Do you understand how to calculate the amount of energy tied up in the rotation of the projectile? It's a tiny fraction of the total kinetic energy of the projectile.
If I remember right going from a 12" twist to a 10" twist in a 2600 fps 308 load will rob something like 2 fps.
Like anything else it depends. Many long treatises have been written about this.
BTW this thread rocks.
