How strong is Grey Buu?

[MDSTSSJ]

All the evil power DID go to Gray Buu. The manga specifically says so. And he's Pure Evil (also stated by the manga), so their's no good influence on him.

But why Grey Boo wears the Dai Kaio clothes?

[Kamiccolo9]

But why Grey Boo wears the Dai Kaio clothes?

Because he split off from Fat Buu, who was wearing them. Gray Buu is a mirror image of Mr. Buu. He was made by Buu expelling all the evil within him, and it took a form that resembled an evil Fat Buu. He went from being Fat, jolly, and pink, to skinny, serious, and gray. He still came from Fat Buu though, thus the clothes.

[MDSTSSJ]

Because he split off from Fat Buu, who was wearing them. Gray Buu is a mirror image of Mr. Buu. He was made by Buu expelling all the evil within him, and it took a form that resembled an evil Fat Buu. He went from being Fat, jolly, and pink, to skinny, serious, and gray. He still came from Fat Buu though, thus the clothes.

Fair enough!

Buff Boo = Pure Boo + South Kaio

Super Boo = Grey Boo + Dai Kaio + South Kaio

Who is stronger?

[Kamiccolo9]

Because he split off from Fat Buu, who was wearing them. Gray Buu is a mirror image of Mr. Buu. He was made by Buu expelling all the evil within him, and it took a form that resembled an evil Fat Buu. He went from being Fat, jolly, and pink, to skinny, serious, and gray. He still came from Fat Buu though, thus the clothes.

Fair enough!

Buff Boo = Pure Boo + South Kaio

Super Boo = Grey Boo + Dai Kaio + South Kaio

Who is stronger?

Buff Buu. He is specifically said to be stronger than Super Buu, because Super Buu still has a bit of Dai Kaioshin influence that occurs when Gray Buu eats Mr. Buu.
Buff Buu is Pure Buu + South Kaioshin, while Super Buu is Pure Buu + South Kaioshin - some Dai Kaioshin influence.

[MDSTSSJ]

Buff Buu. He is specifically said to be stronger than Super Buu, because Super Buu still has a bit of Dai Kaioshin influence that occurs when Gray Buu eats Mr. Buu.
Buff Buu is Pure Buu + South Kaioshin, while Super Buu is Pure Buu + South Kaioshin - some Dai Kaioshin influence.

I knew that Buff Boo is stronger than Super Buu but I wanted an answer like you gave to me. And for that, now I change my mind: Grey Boo is equal to Pure Boo in terms of power.

[SSJ4_Zankuto]

If Dai Kaioshin's heart is responsible for Boo to get his power lowered. Does Dai Kaioshin's heart have any effect on Pure Evil Boo and Evil Boo?

It seems Evil Boo isn't affected by Dai Kaioshin's heart effect. But instead Evil Boo relies on another source of influence that allows him to increase his Ki. When Fat Boo gets mad his power increase yet again probably from same source of influence as Evil Boo did.

Pure Boo and South Kaioshin-Boo doesn't have Dai Kaioshin's heart.

[Mjb1985]

Dai Kaioshin have Super Buu a conscious. He still has a large effect on him.

[SSJ4_Zankuto]

Dai Kaioshin have Super Buu a conscious. He still has a large effect on him.

Yep. The heart of Dai Kaioshin still have effect on Evil Boo.

[OWmyDragonBallz]

Pure boo is stronger than Pure evil boo. The evidence of this is extraordinary.

The only direct evidence we have are their respective fights against Mr. Buu. Gray Buu stomps Mr. Buu much harder and more quickly than Pure Buu does.

All the rest are vague statements that have already been addressed in the last few pages. If you disagree, fine. But don't cite direct evidence when there isn't any.

Comparing fights leads to fan speculation. Pure boo was toying with Good boo. Pure evil boo was serious because he was fixated on absorbing him.
Goku is stronger than Fat boo who is stronger than Pure evil boo whereas Goku states he has to go all out against Pure boo to have a chance and the fight ended up being about dead even.
From this, we can safely conclude that Pure boo > Pure evil boo. Makes sense seeing as how Pure evil boo is most of Fat boo's power, not Pure boo's.
Let's not forget that Pure evil boo is wearing the Dai kaioshins clothing and actually had control over his self conscience. Pure boo immediately blew up the planet one after another.
There is literally no proof that Pure evil boo >= Pure boo and it goes against Dai kaioshin's purpose. Plus that would make Fat boo > Pure boo which makes zero sense. Fat boo reached his limit right when he was about to release the grey boo. Yet Goku still stands by what he says about being stronger than Fat boo.
To narrow it down, Pure evil boo is pure evil yes, but he is still controlled over the dai kaioshin due to characteristics and that shouldn't be taken too literal. Pure boo is stated to be "evil itself". To say that Pure evil boo is >= Pure boo contradicts what Goku said about being stronger than Fat boo who is stronger than Pure evil boo because Goku was dead even with Pure boo. So according to you, Goku is = Pure evil boo who is weaker than Fat boo now? Lol. Like I said, the evidence is extraordinary.

[Kamiccolo9]

Pure boo is stronger than Pure evil boo. The evidence of this is extraordinary.

The only direct evidence we have are their respective fights against Mr. Buu. Gray Buu stomps Mr. Buu much harder and more quickly than Pure Buu does.

All the rest are vague statements that have already been addressed in the last few pages. If you disagree, fine. But don't cite direct evidence when there isn't any.

Comparing fights leads to fan speculation. Pure boo was toying with Good boo. Pure evil boo was serious because he was fixated on absorbing him.
Goku is stronger than Fat boo who is stronger than Pure evil boo whereas Goku states he has to go all out against Pure boo to have a chance and the fight ended up being about dead even.
From this, we can safely conclude that Pure boo > Pure evil boo. Makes sense seeing as how Pure evil boo is most of Fat boo's power, not Pure boo's.
Let's not forget that Pure evil boo is wearing the Dai kaioshins clothing and actually had control over his self conscience. Pure boo immediately blew up the planet one after another.
There is literally no proof that Pure evil boo >= Pure boo and it goes against Dai kaioshin's purpose. Plus that would make Fat boo > Pure boo which makes zero sense. Fat boo reached his limit right when he was about to release the grey boo. Yet Goku still stands by what he says about being stronger than Fat boo.
To narrow it down, Pure evil boo is pure evil yes, but he is still controlled over the dai kaioshin due to characteristics and that shouldn't be taken too literal. Pure boo is stated to be "evil itself". To say that Pure evil boo is >= Pure boo contradicts what Goku said about being stronger than Fat boo who is stronger than Pure evil boo because Goku was dead even with Pure boo. So according to you, Goku is = Pure evil boo who is weaker than Fat boo now? Lol. Like I said, the evidence is extraordinary.

Fat Buu and Super Buu are composed of the same parts. Dai Kaioshin, South Kaioshin, and Pure Buu. This is a fact. It can't be debated. They have the same amount of power inside them, yet Fat Buu cannot access this power due to Dai Kaioshin's good heart sealing the power.
Fat Buu's power increases when he is angry. We see this once against Dabura, and twice against Vegeta. When he's mad, he shoots out steam. Again, fact.
Fat Buu never gets angry against Goku. He never shoots out steam, and he's smiling the whole time. Therefore, he never powers up against Goku.
My theory is that when Buu is angry, the evil power in him allows him to overcome Dai Kaioshin's influence.
When Mr. Satan gets shot, Buu becomes the angriest he's ever been. His anger is said to swell. And since Buu powers up when he's angry, he was growing stronger during this time as well. Why wouldn't he?
He gets so mad, that all the evil ki becomes a separate being. The manga specifically says ALL the evil in him. And all the evil in Fat Buu is Pure Buu. So Gray Buu is made up of, at least, all of Pure Buu's energy, since ALL of Fat Buu's evil went into him. Additionally, some Kaioshin power probably also went into him, since Gray Buu wears Kaioshin clothing. Yet, as he is Pure Evil, there is no negative Kaioshin influence.
Mr. Buu got everything else.
Gray Buu easily dominates Mr. Buu, while Pure Buu has to put up a fight. So Gray Buu is either equal to or stronger than Pure Buu.

Both the steam and the smile. Buu was having the time of his life against Goku. He definitely wasn't mad there.
No change in power was noted, but also, nobody's reaction was shown. Seeing as nobody commented on Gohan's power changing, except Buu, and nobody noticed Goku's power decreasing during his fight until after it had already happened, I don't think this matters. And Goku wasn't watching the Buu's from otherworld; he felt Super Buu and wondered what was going on. Goku had no idea about the whole Mr. Buu/Gray Buu thing.
Vegeta was watching from other world, but no one ever said he sensed them. He was in another dimension, after all. All he knew was that both Mr. Buu and Gray Buu would be weaker than Super Buu, and he had extreme confidence in Super Saiyan 3. But seeing as SSJ3 Goku was unable to stop Pure Buu like Vegeta thought he would, it's clear that Vegeta overestimated SSJ3 Goku. And Vegeta also thought that he could take Fat Buu, which was obviously false, so his power sensing skills are in question in this arc.

As for the Kaioshins, you can't really leave them out of this conversation. They were there the whole time, and their power had to go somewhere. It didn't just disappear when Buu split. And there's also the quote in the manga about Gray Buu getting all of Buu's evil, and, as we find out later, the evil in Fat Buu is Pure Buu. So Gray Buu has to have all the power tha Pure Buu had.

Saying someones' ki has changed could mean that it got stronger though. It could work either way.
They can both represent a power change, but the key thing to remember here is that Buu gets stronger when he's angry. He was having fun fighting Goku. I don't remember if he ever loses his temper in the anime, but in the manga he's having the time of his life.
True, but blast amplification isn't usually factored in to one's true power. Otherwise, Goku at the beginning would have read at 910 by Raditz' scouter, not 416. Amplification comes later. And it could also be implied that Buu was toying with Goku; he's dancing during the fight, and later on Goku says that he underestimated him.

As for Buu powering up significantly, he's made of the same parts as Super Buu, so he should have the same amount of power within him. As he powers up when he gets angry, the Dai Kaioshin influence is overcome, and he can reach Super Buu levels of power, which I think he did in the instant before he split. This would result in both Mr. Buu and Gray Buu being weaker than Full Power Fat Buu, like what the manga says, but it also allows them to be stronger than the initial Fat Buu, who was on Gohan's level.

Even if Fat Buu didn't power up, he still has all of Pure Buu's ki in him though. And all that evil energy was directly said to go straight into Gray Buu, who we know is Pure Evil. Since he is Pure Evil, there is no negative Kaioshin influence, since this was said to be caused by Dai Kaioshin's good heart. Since there is no goodness in Pure Evil Buu, their is no restriction on his power, so he would have to be at least as strong as Pure Buu.

All the evil power DID go to Gray Buu. The manga specifically says so. And he's Pure Evil (also stated by the manga), so their's no good influence on him.

But why Grey Boo wears the Dai Kaio clothes?

Because he split off from Fat Buu, who was wearing them. Gray Buu is a mirror image of Mr. Buu. He was made by Buu expelling all the evil within him, and it took a form that resembled an evil Fat Buu. He went from being Fat, jolly, and pink, to skinny, serious, and gray. He still came from Fat Buu though, thus the clothes.

If you want to disagree, that's fine. But don't go "lol no evidence!" without reading what's been posted. Plenty of evidence has been supplied, and has been more than enough for quite a few people here.