After some further testing with 200 on 200, 300 on 300 and 400 on 400 ASFs fighting each other I can not reproduce my original results. Currently I have a reliable sim difference of at most 1 with 300 on 300 ASFs and beyond.
I've also tried to experiment with damage over time using T1 UEF bombers. In the test I had 100 bombers firing at a Megalith. that resulted in a sim computation difference of ~10.2ms at peak (without) to ~10.4ms at peak (with). Three runs for each. It isn't significant, quite minor. In both cases it aligns with what Ftx and Softles stated.
I'm updating the original post as I consider it misleading at this point. Not sure what happened when I tested before, maybe a background program (World Machine, hurr) interfered with it.