Lots of good advice on this track. Reference the .460 WBY, which I have shot, along with 450-400's, .500NE, .458, and a bunch of other DG calibers is that the .460:
- .460 best ONLY for the most dangerous of game and NOTHING else (too heavy, loud, expensive ammo, hard to get off repititive shots due to recoil, hard for long range accuracy, hell on optics, finish on some models gives off LOTS of glare to make YOU a target)
- Rationale for lugging such a large weapon is negated if you can't hit what you're aiming at. Large game (elephants, rhino, buffalo) can soak up a lot of lead if your shot is poor - even from a .460. In any case, your PH will be there if your shot goes awry, which happens much less with a .375 H&H than the large .40 and .50+ calibers.
- There is no such thing as "too small" if you use the right bullets, hit the right spot, and know how to f/u with accurate shots. Usually the same guns you use in the US for big game will suffice for African game. Deer size and below - 30.06 works great; medium size game (Oryx, Wildebeast, Waterbuck etc - 30.06 - 300 Win Mag, large game only - 450-400 to .458. With that said, the 30.06 will usually kill anything up to Eland if the shot is right, and the .375 anything. There is a lot of logic in the phrase "one gun".
My two cents worth, but of course you hunt with whatever you want to hunt with, and if you want to use a 700NE for impala - you're choice. It's your hunt and your adventure!