No idea how many IRs Fractal is using per cab, but we spent a lot of time doing critical listening tests to ensure there was no way a user could tell whether they were listening to a combination of values that resulted in a raw IR vs. interpolation. In fact, back when it was taking 55 minutes to update to 3.50, we decided to find the ideal number of IRs that would still ensure interpolation resulted in zero negative impact on the sound. We still had to provide thousands of IRs per cab and many tens of thousands of IRs total, but IIRC, it was roughly half of the number of IRs we started with (which is why updating to 3.50 takes about 25-30 minutes instead). Our take is that there was no way to get the same granularity and experience of meticulously moving a studio locker's worth of mics around a cabinet without some interpolation... unless we provided millions of IRs, which wouldn't result in any noticeable sonic advantage anyway.
If our level of interpolation were noticeable, someone would be able to turn Helix's knobs and say "Oh, this is a raw IR. Oh, this is interpolation." They can't.
Conversely, at least one competitor (not mentioned in this thread) interpolates between four IRs per cab. At that level, yes, you can totally tell.
Still, we have all the raw data from our past (and future) cab shooting sessions, including mics not used, mic pres not used, and much longer IR lengths at higher sample rates/bit depths not supported, just to ensure they're relatively future proof. And yes, we also built custom software for our mic robots.
I keep Helix Control on my keyboard controller specifically to speed up editing, even when using HX Edit.
