faster for heavier is right. I have recently dealt with this in a two .308's. savage builds their .308's with a 1-10" twist, remington for the most part uses 1-12". i started out with the savage and couldn't get anything to work until i went to a boattail bullet, 165 g and up. even with a 165 or 168 g bullet, it still shoots better when i slow it down a little. the shape of the bullet matters too. higher BC bullets seem to need more twist to stabilize than lower BC bullets.
my remington, on the other hand, shoots anything 168g and less like a house on fire. it is much less picky. go over 168, though, and it doesn't like them.