Ok guys I am from Missouri and going to call this one.
I will concede that when Remington released the 416 Rem they had issues with pressure. I think they were using a ball powder similiar to W-760 and it was an issue. But this is a case that has over 95 gr capacity, and with canister powders loads are 76-78 grs with a 400 gr bullet. Remington long ago corrected these problems, and this is really old news.
I spend a fair amount of time over at Accurate Reloading. If there is a site that has more PH's on it I sure haven't found it. Do a search there are no posts to support this. And there are a lot of PH's and African hunters that post there, and they would be talking.
If I am wrong on these I will concede the point, but I really need more than I saw it somewhere on the wild-wild world of the web.
I will also concede that most PH's aren't using it. But the choices of these guys are more along the lines of 404 Jeff, 425 Westly Richards, 458 Lott, 470 Capstick, 470 Nitro Express. It is an interesting study if you really look at what some of these guys are using, they use some very old and valuable rifles a lot of times and have had them for years. But pay attention to what they are buying on new guns. And I will add that if I was going to buy a gun specifically for an African hunt the 416 Rem would not be my first choice, I would pick from the above list. But for most American hunters that might once go to Africa, and will want to use the rifle on something other than paper punching the 416 Rem isn't a bad choice.
I will add my experience is not first hand, I need to rely on guys that are doing it and reading. I tried unsuccesfully to convince my wife to a Zambia hunt next year, and its been file 86ed, but I was planning on either a 404 Jeffrey, a 416 Rigby, or a 45 cal, to supplement my 375H&H.