Some of what follows is based on comparisons, and some on some relative scaling...

Deku now, at 45%, is significantly faster than Gran Torino, someone who could trivially, casually blitz him when he was back at 5%.

All Might, who had one of Deku's stack of powers, albeit at twice the level that unprepped Deku is rolling at now, has blitz scenes where he KO's a stack of villains across an area where nobody even sees him move. He's standing on one side, then he's standing on the other, and all of the villains are down, nobody has seen him move.

All Might also has that feat against the first Nomu of hitting it... something over 300 times at his max level there, IIRC, in a tiny fraction of a second, overloading the Nomu's ability to absorb. That's massively more than bullet-timer to me. Spider Man or Danny Rand aren't punching full force 300 times in a fraction of a second, and those guys are pretty much the tippy top of "high end BT."

Given that Deku can now use 45% of OFA's stat boost, it would seem like he should be capable of 45% or so of his performance without resorting to faux 100% / Fa Jin.

Deku, Bakougo and Todoroki specifically trained speed with Endeavor, who was so fast when they started that he'd crossed a huge distance and blitzed their foe before the boys could really react or do much. Deku then shows, a short time later, that he's at the very least a good bit faster than Endeavor.

Yes, Fa Jin / faux 100% took some on-the-fly, mid-fight "prep" but he outran a super bullet fired from a super powered arm gun wielded by someone who never misses out to 4km. That's way, way beyond bullet timing to me, which tracks based on All Might's feats, using, presumably 100% of OFA himself. And half of that (Deku's unamped 45% OFA) should thus also be way, way above simple bullet timing.

Yes, calculations and scaling aren't the same as feats, and maybe it's a bridge too far. Sadly, or not, MHA doesn't really deal with gun battles too often, and it also gives some level of superhuman reaction speed to people with quirks where they really don't make sense. If the consensus is that we shouldn't transfer or calculate Deku's stuff that way for now, I'm not going to die defending that bridge.