But I want to talk about dear, boar, black bear, elk, sheep...the "big game" that doesn't have that invincability aura around them. The main advantage I see in moving up in caliber beyond .30 is throwing a heavier bullet without having to make them 1.5" long.
For those animals there's no reason I can think of for anything beyond 30 caliber. Larger bullets can often increase penetration and on larger targets this can be important. The various .300 magnums (and even the .30-06) will handle this group of targets nicely.
To the question about twist:...yes length of the bullet is the determining factor.......not weight. Further slip don't happen in the barrel.....your theory on the twist rate for longer bullets might be absolutely correct. A study of gyroscopic motion will be need to prove it but I'd suspect you're spot on there.