I don't usually get involved in all this technical matter regarding pressures and this and that when it comes to picking a suitable rifle for Africa. For DG take anything with more than 4000ft/pd energy and for the plains game you can take any medium sized caliber.
The last thing to worry about is all the tech info. It makes no differance to the animal.
Most important is that you are comfortable with your rifle and that you can place proper shots. I'd rather hit an Impala on the right place with a .243 than the wrong place with a 375H&H.
I have a couple of different calibers and my theory is that if I can't shoot it with my 243 I'll shoot it with my 460wby mag and it makes perfect sense.
If ever coming to Africa, don't go around looking for a suitable rifle just bring your trusted one that you usually hunt with.
For example if you own a .375H&H and you have been hunting with it for years then don't go buy a 416 or 458 to come and hunt DG in Africa. You'll do much better with your old 375.
Remeber it is your safari and you must be comfortable. You are the one who is paying the fees at the end of the day so make sure that there is no doubt when you aim that rifle.