The faster twist is meant for heavier bullets. Heavier bullets=slower velocity, and thus require faster twist to stabilize them. With the lighter(faster) varmit bullets that fast a twist rate can literally cause the bullet to come apart. The 1 in 9" twist is a compromise, military rifles use 1 in 7" twist for standard 62gran ammo. Most bolt guns in .223 will have 1 in 12" twist, and I have seen one in 1 in 14" twist.

There is a point at which increasing barrell lenght won't provide any increase in velocity. From what I have seen, 22" is a pretty standard length barrell for .223rem in 1 in 12" twist.

Good luck