My experience is only with Asiatic buff, but I believe that over .40 bullets (at nitro-express velocities or better) usually result in the animal clearly registering the hit. Under .40, and a big bull will often recieve the bullet with no visible signs of a hit, and run off. If placement is good, they may die just as quick, sure, but I prefer to actually see them recieve the bullet.
I like to hear it too, of course, but often the shooter under recoil doesn't hear the impact.

I assume this 'over .40' thing would occur with Cape Buff as well.

..and I agree with 500Nitro, that the .500 NE is another major step up in power, as most of the old-time meat-shooters in the Northern Territory (Vic P_, Dave L_ et al) will attest.

I'm certain that the 'over .50' thing would occur with Cape Buff as well!