I've killed four elk, which doesn't give me a huge data base; I have only used one round per elk, however. My first was hit with a factory Remington "trapdoor-safe" 405 gr. soft point, fired from a Sharps carbine repro. If memory serves, this load advertised about 1400 fps. It was a quartering-away shot at about 85 yards. The bullet went through the heart and broke up the off shoulder, coming to rest just under the hide. He was dead before I could walk up to him. The next two were killed with a .54 muzzleloader loaded with a patched round ball and 110 gr of FFg, both broadside and within 70 yards. Neither required a finishing shot. My most recent was with a sweet, vintage Winchester M70 chambered in .300 Winchester. I chose the first factory-loaded Barnes "X" loading I ever saw for the hunt, and bought all five boxes the shop had. After practicing for 300-yard shots, my cow jumped up 50 yards from me, again broadside, and I stuffed the 180 gr. bullet in her heart. Of all four elk, she's the only one that ran at all, though she only managed a hundred feet or so before succumbing.
One of my partners on the first hunt used a .338 H&H on his elk, and scored three boiler room hits before it went down. I helped dress the elk and am convinced that any of the shots would have killed that animal, but he just got a little trigger happy when the bull didn't get picked up and slammed to the forest floor. Dad got his elk that day with a 30-06 firing Remington 180 gr. round points, one and done, no tracking.
What I take away from this is that using enough gun is essential, and that using more gun than necessary is just fine, but does not relieve you of placing the bullet correctly. I would feel perfectly comfortable using my 6.5x55, as long as I am able to place the shot the way I want it placed. Using the 6.5 would make it doubly important for me to avoid full-on shoulder shots, but it would be capable if I did my part. That being said, I'd reach for any of the other rifles I mentioned above, given a choice.