Regarding XTP's, my experience has been different than what some have suggested here. I have used the .475 400 gr. XTP on a South American water buffalo much heavier than any cape buffalo I've shot, with complete success. I 've used that same one on bison, bull elk, a couple of large bears and several wild boar.
Ive used the 350 gr. XTP from a .500 WE on a couple more bison and boars, once again with no problems.
Regarding the .454 XTP's, there's a big difference between the original version and the thicker jacket Magnum version. That Magnum bullet is much better for anything of significant size, in my opinion, although I personally wouldn't use any .454 on buff, if I had a .475 or .50.
Having said this, I don't take Texas heart shots, and habitually take heart/ lung shots on game, not shoulder shots-- where a hardcast bullet should prove superior on penetration.