A 6.5 will kill a fired up older bull, no question. The difference is how far he goes before he realizes he's been killed. Adrenaline does some funky things. If he can cover 150 yards in steep black timber you'll probably never find him. If you do find him, he'll be upside down in the water at the bottom of a steep, ugly ravine.
That said, if I could afford it I'd have a Barrett Fieldcraft in 6.5 Creedmoor on order for a steep country elk rifle. Until then, I'll have to keep lugging the .375 H&H. Like any tool, you just have to know its limitations and work within them. Use the heaviest bullets that will shoot well, don't take ridiculous shots, and don't listen to anyone who says it isn't a good elk rifle.