Buffalo are tough but not really all that hard to kill. In fact I know more than one African PH who's killed many of them with a .30-06 or something similar, even big bulls, legal when doing "control" shooting, whether of animals damaging crops or just thinning herds in parks. My friend Kevin Thomas has killed quite a few with a .30-06 and 180-grain Nosler Partitions, a load that some American hunters think is on the light side for elk, and Kevin had no problems.
There is, of course, a world of difference between hunting and culling -- and a hunted buffalo is not the same as a culled buffalo. The former is often stalked and spooked, stalked some more, spooked some more, and generally brought to a state of attention mingled with irritation as it makes its way through increasingly dense cover -- by the time that first shot is fired, the buff is both aware of the hunter and in possession of certain terrain advantages. And almost without exception, that shot is taken from close range where things start happening very, very quickly as that first muzzle blast echoes through the bush.
By contrast, culling operations typically involve longer shooting, jacklighting at night, and resting the rifle over the rear view mirror of the truck. This isn't my description, but rather a general representation of how it was described to me by the handful of African PH's I've met who've done serious culling work.
I completely agree that a .30 cal bullet in the lungs is going to kill a buff quite dead. Eventually. From the safety of a vehicle, there would be no reason to use anything bigger, especially when shooting a buff that's oblivious to what's going on. But as for stalking through the jesse, bumping an old dagga boy at 30 yards, and having to take a quick shot and be ready for whatever happens... Well, everyone's entitled to their opinion. Personally, I think it's madness to be *hunting* any species of dangerous game with a rifle that's insufficient to drop it RFN with a shot directed at the CNS while the animal is bearing down on you in a direct line. I do more hunting with the 30-06 than with all other calibers combined, but I have absolutely no illusions about the odds of it quickly stopping a head on charge from a buff. And while that's actually quite a rare situation to be in if you're hunting sensibly, it's still the one you have to be prepared for.
Admittedly, my own experience in this regard is quite limited. But having put a 300 grain A-Frame from a .375 H&H through the front of one lung, through the major blood vessels along the top of the heart, and through the back of the other lung ... only to have Mr. Buffalo take off ... well, that made quite an impression. Despite that and a second shot that also connected, it was still standing and getting along just fine when we caught up with it 20 minutes later. Was it a dead buffalo at that point? My guess is yes. But it still had plenty of gas and was quite capable of ruining anyone's day.
As it stands, I do think that the .375 is a great rifle for buff. But I don't think that's what I'd choose if hunting alone and without a PH to back me up. Dogleg's got it right, I think, with his growing affinity for buff rifles that start with the numbers 458.
Well, that's my 2 cents...