Technically, the faster the rate of twist, the faster the round SHOULD be
BUT, I think the difference is nominal unless you go from 1 in 14 to 1 in 7. Obviously, a change that big not good unless your shooting small caliber (5.56). I don't think it makes a REAL difference, maybe an average of 10 fps if you went from 1 in 9 to 1 in 7 in an AR. Before anyone beats me over the head with this, it is purely speculation. I have no proof to back up the numbers, I'm just doing something that will probably make me look dumb and speculating.