Recreational Sport hunters do not need a gun this big. most PH's don't need a gun that big.
A quality 375HH shooting premium bullets like the TSX at 270 grains or more is plenty and easy to shoot. I've not met but a few folks that can shoot a 458 or bigger accurate, and the 458Lott is a Whole lot more then the 458 Win mag. Shooting from kneeling standing prone, etc Anything is possible in the bush. Any slip up in shooting with that big Lott is gonna be far more painfull then having to shoot two times with a 375HH.
If it were a hunter of mine asking what to buy for an upcoming Buff hunt I would tell them to get a 375HH, or just use mine when they arrive.
If a fella just has to have a big thumper of a rifle to fool around with that's a completely different issue. No Recreational Sport hunter needs more then the 375HH with a premium bullet, it's accurate easy to shoot, easy to practice with and works like a dream. If you really want to get a bigger gun the 416 rem mag is the logical choice. If your question is about the single shot, that's a fine idea, but your PH will be far more likely to follow up knowing you have only one in the tube.
I'm rather practical in nature, I would not be buying any firearm to use on a one or two time trip. I would sooner use that money for more trophy fees and other gear.