Ummm... Of course fast rolls and mid-rolls have the same amount of iFrames. The difference is the distance traveled, and the amount of Frames in which you are vulnerable during the second half of the roll. You aren't going to gauge the benefits of going under 30% while rolling into a wall, you're purposely making yourself vulnerable. On top of that, none of outside variables are controlled. It probably does take at least 0.1 Poise for you to be able to Poise through attacks during rolls, just like Hyper Armor does. But the difference should be in how many consecutive hits it would take for you to stagger during your rolling animation's vulnerability Frames, not if you can avoid getting staggered at all.
From what I saw, the only way you might roll into a wall endlessly and take a long time to get staggered, during a roll, is with 41+ Poise. The point of Poise has always been to offer resistance, not invulnerability from "breaking form." If you're trying not to get staggered at all, in association with Poise, I doubt you'll find what you're looking for, not without a cheat engine anyways.