To me 20" is the perfect length for a 308. I assume 7-08 will be very similar. As far as velocity is concerned you have to go to 26" or more to see any meaningful gains. In fact, my 20" Tikka CTR 308 shoots the same ammo FASTER by a small margin than either of my 308's with 22" barrels. I find 20" easy to carry through brush, but it isn't short enough for noise to be a problem. Maybe in another cartridge.
I wouldn't go any shorter, and 22" doesn't bother me either. I wouldn't pay to have a 22" barrel cut shorter. I might if it were 24". But if building I'd want something shorter that 24" with a little meat in the barrel. Split the difference and go 21"
