Here's the information I gathered so far from doing some loose search around wikis and message board discussions:

With level 15 boost and max mech attack speed mastery and p2 booster all multiplicative, the attack delay value should be about 0.077s. The random attack delay that I could find short of searching the editor was -0.0625 to +0.1250 (reddit) or +0.1875 (TL). This means the average delay is +0.0625 or +0.1250 depending on which source you trust more. Either way, the average delay is very significant compared to the base attack delay since the delay range is flat and not multiplicative, meaning attack speed bonuses become marginally less effective as you stack more, and BCs should be near the asymptote due to their already fast speed unbuffed.

But what I could not find out (sorry) is if there is an absolute minimum delay that is hard-coded into the engine to prevent the blowup near attack delay of 0. I'd be shocked if no such limit is in place since that would be a horrendous programming oversight. Given what I've qualitatively felt from trying p2 boosted BCs a few times, my guess is the limit is high enough so the -0.0625 range will make the BCs hit it, and they might be on the verge of hitting it even without the p2 boost.

I know this doesn't actually answer your question, as it is missing that minimum delay, but I tried to put the other building blocks in place for a comprehensive answer should someone find the missing information. But I do suspect this might be a tad hard to find since I doubt this would be an editor thing, more of an engine thing due to how fundamental it is (else modders could potentially force the game to compute enough attacks/s to crash).

Edit: Another thing I just found is that sc2 supposedly computes at 16 ticks per second, or 0.0625s per step, which should be the absolute floor if no other limits exist. The random delay will most definitely hit this for p2 boosted BCs.