First of I'm against over rpm'ing bullets. The faster the twist rate the more resistance to the bullet traveling down the barrel. So you get higher pressure and decreased velocity.
The other fact is that higher velocity decreases the need for low number twist rates. Example a 16 inch barrel .223 can shoot a 55 grain bullet fine. Take the same bullets and put them in a 20 inch barrel and they fly through the paper sideways at 100 yards because of the velocity increase.
Shilen sells a 27 inch 9:1 257 barrel or at least they used to. That works very well for my buddies 06 and 115 Bergers. The 121 is longer, but might work fine going faster.