Think about it, why would Toriyama first Make Goku tell Vegeta that Evil Boo ki is rising then make Kaioshin react in totally opposite way compared to Goku and Vegeta and then make Kibitoshin give exposition on the whole Story and mention decrease in power?Chapter: 508 (DBZ 314), P4.2-7
Elder Kaioshin: “…So he was finally completed…Which is to say…that this current…small Majin Boo…is the initial…mo…most troublesome one…”
Kaioshin: “….Yes…the heart which he gained by going so far as to lower his power through absorption(s) …has returned once again to the way it was…he has no self-control whatsoever…he has become evil itself…”
How's that not enough to tell That Pure Boo is above Evil Boo? Goku and Vegeta clearly prove that they both underestimated Boo.
Their reactions to his blast is something else, i don't expect these reactions from someone who just now thought they maybe able to manage something :
Chapter: 508 (DBZ 314), P8.7
Context: Boo forms a big ki blast
Goku: “It-it’s huge…! You’ve got to be kidding…! Do-does he intend to unleash that…!? …We can’t knock back something like that…!”
The reason why Goku wants to Fight Pure Boo one-on-one is not because of Ki going down, it's the same reason he wanted to fight Vegeta and Final Form Freeza alone. It's his desire to test his powers and warrior pride of fighting Unfused character without fusion, otherwise in the entire manga why would they not make a comment on Pure Boo's ki going down when they made comments on ki everytime:
Notice how old Kaioshins want him to use potara so badly and screams at him and calls him out. Kibitoshin has given up on telling them anything at this point because they never listened to him earlier before Boo was revived and it's same thing here. Goku never implied any fall in Boo's ki like something say "Don't worry his ki is way lower than what it was when we were inside him", there is hardly a mention of Power here. On the contrary, Goku mentioned "Dangerously" factor and even then he's trying to come up with a Plan while Pure Boo is destroying planets.Chapter: 509 (DBZ 315), P6.8, P7.1-5
Context: after Kaioshin hands Goku his Potara, but he refuses it
Goku: “…Like I thought, these…just ain’t suited for us…Even though you went out of your way to hand ‘em over…We want to fight with only our own power. I’m sorry, especially since things are so dangerous now…But [Boo] ain’t merged anymore either…“
Elder Kaioshin: “Yo-you idiot! What are you saying?! And at a time like this! It’s not like you gu-guys are in a martial arts match with Boo, you know!”
Vegeta: “…Well said, Kakarot. Just like a true…Saiyan.”
Goku: “It’s alright. I’m tellin’ ya, don’t worry. He can’t come all the way here. We’ll think up some sorta strategy. I feel bad for the aliens who will be sacrificed in the meantime, but we’ll use the dragonballs later…”
Also, here Pure Boo sensed Them and then reached them which means he has the abilities of a martial artist and it's very very likely that when he first appeared on earth He was suppressing his Ki, i think it should be obvious, so it's not like Goku and Vegeta judged him based on his size, Pure Boo has shown these skills related to Ki. It's typical Toriyama gag of atleast one character mocking the final form of being weak and other correcting them or reacting opposite of it. Kuririn did it with Freeza (don't tell me Kuririn can't sense ki) only to be corrected by Piccolo, Vegeta tried to do that (don't tell me Vegeta can't sense ki) only to be corrected by Cell when he went right through him. It's same thing here, Kaioshin is reacting totally opposite to Goku-Vegeta and then goes on giving backstory on him and mentioning "Power". Which means Good Boo inside Evil Boo was Kaioshin influence from both Grand and South Kaioshin. And no, Boo can't access Southern Kaioshin absorbed ki because he's detached from the body so that skyrocketing ki has to be of Pure Boo. Remember How Evil Boo reverts to weaker forms the instant someone is detached from inside? When Goku and Vegeta detached the kids and Piccolo he changes instantly and drops ki instantly. Whereas here, it skyrocketed and turns into a drawn out transformations which is typical Toriyama for villain Transformation.
This is after Goku and Vegeta finally see Pure Boo in action and their conclusion :
Chapter: 510 (DBZ 316), P13.5
Context: after Super Saiyan 3 Goku explains he needs to gather ki for 1 minute to defeat pure Boo
Goku: “Dammnit~~~If it was with the Potara, I could have done it in one blast. …..Cheh~~~I guess I went and showed off too much. But I thought things would go a little better than this…!”
Chapter: 510 (DBZ 316), P12.6
Vegeta: “The truth is you were right [that I’d be killed by Boo]. That Boo is stronger than I imagined…And so are you, Kakarot…”
Chapter: 507 (DBZ 313), P12.4-5
Context: after Goku and Vegeta revert Boo back to regular evil Boo
Goku: “Hehhe~~eh! With this, Boo’s power should have fallen significantly! We’re almost there! See, see: the size of his ki is completely different than before!”
Vegeta: “Alright! Let’s blast out of here and escape!”
Goku: “Wait! Even though Boo has returned to normal a whole lot, we’re still simply no match for his strength! If we go outside like this, we’ll definitely be done in…!”
When people talk about Goku saying he's no match for Evil Boo, Goku actually uses "we" as if he's not even considering Ssj3 and he says that mainly to persuade Vegeta to fuse. If one try to read them in order, Goku first notices how fusion achieved by dancing is still working inside Boo's body (unlike Potara magic) when he saw Goten and Trunks then he realized that with Fusion there is still a chance. After he made the comment of them both not being a match for Evil Boo, Vegeta asks what does he mean then Goku called him out for destroying potara, then he tried to tell him about fusion like a Surefire way to kill Boo based on Vegeta not knowing about it "but there's another way" but Vegeta replied with him knowing fusion which surprised Goku. Notice the gag faces Goku makes here, it's hard to believe he really was that afraid but we know for a fact that with ssj3 Goku always had unwillingness to use it or not and always preferred fusion over it. He tells similar lie to Kuririn and then Piccolo about not being a match for Chubby Boo (before Separation) but later he admits that he could have beaten him. And incase, Goku went to fight him and Evil Boo was far above Goku like he said then the scenario with him would definitely play out the same as with Pure Boo.
Goku saying he needs to gather enough ki for a minute is same as Piccolo's Makankosappo, it's not that Goku is above Pure Boo but to wipe Boo out he thought he atleast required Ki gathered for 60 seconds. We already saw characters producing ki blasts far stronger than them like with Piccolo's Makankosappo vs Raditz or Tenshinhan's Shin Kikoho against Cell. It's not different here.
Also, it's clearly implied that Gohan was no match for Pure Boo alone, Goku wanted both Gotenks and Gohan to fight at the very least and he was just trying to guess what Vegeta was upto.
Goku : “Oh, I know! You’ll bring Gohan and Gotenks back to life so they can fight.”
The way it's worded highly implies both are required to manage something against Boo.
Vegeta would never prefer unnecessary risky solution if Gohan is enough. Gohan couldn't beat Pure Boo, otherwise, if Gohan was strong enough, Goku would have found Vegeta's idea stupid and wouldn't have agreed.
Goku also recommended Gotenks, we all know his Time Limit for ssj3 is 5 minutes so calling him is completely unnecessary unless it's absolutely necessary, we have already seen how naive Gotenks is as a warrior. So if Goku wanted Gotenks, Gohan, Vegeta, Good Boo to fight together to take Pure Boo down then it totally makes my point clear. Narrative-wise Goku didn't know how to finish Boo so he was guessing that Vegeta is trying to gather as much strong warriors as possible but after Vegeta told Goku that he's going to take every ounce of ki from everyone, Goku was happy with his plan which means Pure Boo is way above Gohan.
Again, Goku was never a match for Pure Boo, he himself realized that he's far outclassed by Pure Boo that's why he thinks he needs to train to catch up.
Goku : “And if worse comes to worse, we can just fight again. Let’s train so that this time for sure we won’t lose even if we go one-on-one.”
This proves that Goku thinks no one is enough to deal with Pure Boo and if he thought Gohan was enough, he would not even say this but say something like "don't worry we already have Gohan to deal with him incase the monster emerges again".
Also in an interview, Toriyama told how the final forms of his villains are least dangerous looking and shorter compared to previous forms. Infact, in an interview the interviewer directly asked him with the example of Pure Boo being the strongest and he said yes and answered it.
Maybe he's not as strong as Gohan-Boo but what i can assure is he's definitely stronger than Evil Boo and he wrote the whole final battle on Kaioshin realm with this in mind, otherwise nothing makes sense, then worrying about universe getting doomed if Genki Dama doesn't work doesn't make sense, Goku calling earth person fools for being ignorant and saying the earth will get destroyed again makes no sense if Gohan is there, everyone giving their ki from Afterlife, Kaioshin realm, Namek, Earth, etc. and praying that Genki Dama works doesn't make sense, Kaio or Piccolo or Old Kaioshin not calling out Goku for not asking his supposedly "stronger than Pure Boo" for help doesn't make sense, Vegeta fearing that Pure Boo will emerge again from Good Boo one day doesn't make sense if Gohan was strong enough to kill Pure Boo, literally, it's like wanting the Boo arc climax to look awful, if one thinks Evil Boo > Pure Boo, when Toriyama is better author than that.