I think, with the Impact, this would depend greatly on the amount of time the owner has spent fine tuning their other settings.
Every setting will effect another. So, someone shooting with factory settings will, most likely, have different results from someone who has fine tuned their settings. The regulator is a major player in SD, and in this equation, and its performance can be masked by the other settings.
That being said, I had outstanding SD's with my Impact. IIRC, The best I had was an SD of 1 while shooting .30's.
Cheers!
Tom