How strong would jiren be if he achieved ultra instinct?

[Ssjcell]

How strong would he be stronger than the grand priest?

[Mister_Popo]

We don't actually know what the actual gap is between a very strong God like Beerus / MUI Goku and the angels / Grand Priest.
We had the original BOG-movie scale Goku 6 / Beerus 10 / Whis 15, but that has been retconned in Super.

Logical reasoning would insinuate at least angel-level, but the Grand Priest, man, we don't know how much stronger he is above the rest of the angels.
He is quoted to be one of the five strongest fighters in the Multiverse for the anime and the strongest for the DBS-manga.
The numbers would be astronomical to say the least.

My headcanon answer would be he'd be stronger than Whis, but not as strong as the Grand Priest.

[Lionel]

Daishinkan blocked two serious attacks from beleaguered Hakaishin with his fingers. Impressive, but we've seen other characters outright resist blows directly to their anatomy without any kind of reaction.

Because of how drastic the boost in power was for Goku, I'm going to say Jiren would surpass all of the angels including Daishinkan -- that guy is a monster. Far as know, he's the only ningen to surpass Hakaishin without the need for god ki.

[Polyphase Avatron]

I would think that he already has at least some elements of it. We know Beerus can use it to an extent so Jiren might too.

[Mister_Popo]

Daishinkan blocked two serious attacks from beleaguered Hakaishin with his fingers. Impressive, but we've seen other characters outright resist blows directly to their anatomy without any kind of reaction.

Because of how drastic the boost in power was for Goku, I'm going to say Jiren would surpass all of the angels including Daishinkan -- that guy is a monster. Far as know, he's the only ningen to surpass Hakaishin without the need for god ki.

Yes, but we have to take into account the angels probably are more ancient beings than the Gods.
They have practiced the art of self-movement potentially since the beginning of existence.
Moreover they probably have mastered techniques that are way superior or even more advanced than UI within this timeframe.
The gap between them and the Gods could be much larger than we think.

The Daishinkan however is an different ballgame since he basically is by far the strongest martial artist to date mentionned in the Multiverse (with notion we don't know who the other four are, where they are compared to the GP).
Its stated that his powers dwarfs Whis's comparatively. When Whis sparred him once, he "couldn't even get to his feet."
He is known to master extremely advanced techniques like portal creation and magic materialization.
He could very well end up farting UI Jiren out of existence.