In answer to your original question . . . . the affect of having more twist than you need is usually a slightly less favorable pressure/velocity ratio. A 14" twist .22-250 might be able to get 3750 fps out of a 55 gr. bullet before it shows any pressure signs, and a 10" twist might start showing pressure signs at 3700 fps. And, at least in theory, the closer the twist is to optimum, the better the accuracy will be. In practice, this seems to be whatever the individual barrel likes.

The only cardinal rule about twist rates is that if you don't have enough, you are absolutely screwed - no exceptions.