The majority of wounded buffalo that manage to hurt or kill people do so because the client failed in his only real responsibility - placing the first shot in exactly the right spot. A 400 gr .416 is no more lethal in the wrong spot than a 300 gr .375. I would further argue that on an in-bound buffalo, neither the .375 nor the .416 represent stopping rifles. That buffalo will have to be either brained or spined in the neck with either. It is one of the arguments made by one of the most experienced PH's I know in Moz whose "back-up" rifle is a .375 (rather like that Don Heath fellow and his 9.3x62).
The average DG hunter going to Africa will hunt one buffalo. A few will hunt a couple. A very small minority will hunt multiple ones. I would suggest that first time client is best armed with the rifle and sighting system that is the easiest and most natural transition from the deer rifle he or she uses at home. I do not believe anything beats a scoped .375 for that job - unless perhaps a 9.3. In other words, with a .375 a client is more likely to do his one job successfully than with something else.
If we then consider client accuracy and confidence in his first shot, that makes an apples and oranges comparison of the .375 and .416 and any "obvious" conclusions rather suspect.
One other point. I have owned a number of forties. I currently own a .404, 500/416, .450, and a .470. I shoot them all very accurately. When traveling internationally on 10-14 day hunt, the last thing I want to do is schlep multiple rifles. In my experience to date, nothing does it all better than a .375.
Finally, you chose your avatar. I assume you expected a reaction. You have had a couple. It looks like someone trying to express their inner Mark Sullivan. Sorry if I drew the wrong conclusion. Nós somos quem fingimos ser. (Kurt Vonnegut)