I shoot a CZ 550 416 Rigby and 416 Remington on the Winchester 70. I like the Rem version of these two better for handiness and was quicker to my shoulder. On my one buffalo hunt (I'm not an expert) i used the 350gr Barnes TSX's at 2550 fps with R-15. You can get another 100 fps out of this quite easily and i did, but backed it down for a little less recoil. I carried that rifle maybe 40 miles on a 14 day hunt. When i finally shot my buff at 15 yards, it was instant lights out for the buff, fell on its nose and death bellowed.
All this to say, make your rifle balanced between power and handiness, and you certainly don't need more juice than the 416 Rem Mag provides. My PH carried a stock 416 Weatherby Mag, but complained about the price of ammo in Zimbabwe to feed it.
