I agree with DM and Bigbulls.
I think alot of people put way too stock in Kinetic energy and not enough in bullet design.
I took my son to the zoo a few weeks ago and eyed up some of the 'big game' there. From the looks of things in real life, there is no reason why a properly loaded 30-06 couldn't easily take a Grizzly, though I wouldn't consider a .375 H&H to be overkill either. Ditto with the Hippos and Rhinos, though I think I would err on the high side of a .458 winchester magnum because they could stomp me into instant pudding if my shot wasn't perfect.