Sounds a bit optimistic to me, since the "improvement" in the 223 isn't all that great. You're already dealing with a case with fairly minimal body taper and a reasonably sharp shoulder. In 223 match rifles, 2,750 with the 77s is already moving pretty good and flirting with pressures. I ran this one (223 AI) out on Quickload with both a 22" and 26" barrel, and went from 2,800, on up to about 2,900 fps. That was with the cyber-pressures running at absolute max, and I doubt I'd be pushing a real rifle quite that hard. Especially when it was my face behind the bolt, if you get my drift.
If you want 3,000-3,100 fps out of the 75s, I'd say a bigger case is in order, not just a longer barrel.