Within relatively close range, with appropriate loads, it should be adequate. I would say adequate would be a .750" RB, hardened with antimony & a bit of tin to about brinel 13 or 14, along with a decent charge of powder, say 7 to 9 drams.
27.3gr. to the dram.
A ball of that size should give some cavitation through the lung/heart area. That is, it should make a hole larger than it's diameter or perhaps the same size, ie: .77", through that medium.
If single shot, you should have some sort of backup - maybe even if SxS.
I've never killed nor hunted a Cape Buffalo.