Not necessarily. I shoot all my forties extremely well - every bit as accurately as one of my .375's. If I were going on a pure buffalo hunt, I probably would take one of them. However, I haven't done that hunt yet, and am unlikely to do so. All of my buffalo have been taken while also hunting other game on the same safari. Nothing offers a more practical solution for such a mixed bag hunt than a .375. These days, it will be loaded with A-Frames, and I would be just as confident trying to stop a buffalo with it as I would my .404 or 500/416. My .470 would likely do a better job turning an animal, but it is of limited utility for many other scenarios. So, I consciously do not use the largest caliber that I can handle accurately because it is not necessarily the most practical.As a theoretical question, you will get many answers, but as a practical question, I just have to agree with @rookhawk , just use the largest caliber you can handle accurately.
From what I have seen, and heard from PH´s, most "african hunters", cannot shoot properly a big bore rifle, and much less a double.