It's a myth that any significant velocity is gained with a slower twist, but recent research has shown that a faster twist will result in a higher BC if the "over-stabilized" bullet has a stability factor (Sg) of 1.5 or over. And some 168-grain .30 will not get to 1.5 with a 1-12 in some conditions.
I have yet to own any .30 caliber from .308 Winchester to .300 Weatherby with a good 1-10 twist barrel that didn't shoot 165-168 grain bullets very well.
