GeauxTide doesn't answer my question about longer barrels getting higher muzzle velocities, but instead states:
I chrono 2920 from my 22" 788 in 308. What does 80fps give me? The OP was talking about hunting rifles. Who wants to lug a 30" barrel bolt gun hunting?
I'll assume you're using 155-gr. bullets as that's what I referred to in 30 inch bullets. I think the peak pressure it gets to push a 155 out at 2920 fps is way, way over safe limits. So. . .
What load does your 788 use that gets 2920 fps with a 150 or 155 grain bullet?
Meanwhile, answers to your questions. . .
An extra 80 fps gets you 3000 fps. Enough to keep those light weight bullets supersonic through 1000 yards with excellent accuracy. Someone hunting with a .308 Win. and 150-gr. bullets wanting 3000 fps muzzle velocity, safe peak pressure and excellent long range accuracy will use a longer barrel.