Yes sir, your right about twist rate. My mod 600 wouldn't shoot 200 gr bullet's very well but shot 180 gr bullet's as well as the 165 gr bullet's. My understanding is that the twist should work for the heaviest bullet and should be alright for lighter one's. Yet with that in mind, I never was able to get a 150 gr bullet to shoot in that rifle.
I think that the optimum bullet for the 308 is the 165 gr because, it will hold together as well as the 180 gr, penetrate more than enought on anything you'd normally use it on and give a bit more velocity than the 180 gr. Now when I lived in Alaska and carried a rifle in the bush, I carried that 308 with the 180 gr bullet, not sure it was really necessary.
Never was in a position where I needed to find out!