A couple of comments.
Do not take more than two rifles. You'll just get confused at the wrong moment.
Do not take more than one weight of bullet in a .458. My Model 70 shot 6" different in elevation at 100 yards with 350 and 450 TSXs, which I think was due to recoil while the bullet was in the barrel. The 350 grain TSXs were going 2700 fps and the 450s were going 2200 fps. The only way to get the same POI was to slow them down to the same velocity as the 450s, and there's no point in that.
The 450 grain TSX works fine with buffalo. You should probably also take some 450 grain solids for the second or third shot. Ask your PH.
Don't worry about getting "a little" extra velocity or slightly smaller groups. Instead, practice shooting off sticks.
I experimented with 350 grain TSXs and don't think it matters what crimping groove you use. Just make sure they fit in the magazine. 2200 fps with a 450 grain bullet works just fine. Don't overthink things.
I took a Cape buffalo and a bull elephant with a .458 on my first trip to Zimbabwe. The second trip I took a Cape buffalo and elephant with a .375. I don't think it made any difference.
Don't worry about compressing powder. That's an old wives' tale. For years I shot the 1000 yard matches at Camp Perry with compressed loads. It gets hotter at Camp Perry in August than it does during most of the hunting months in southern Africa.
