Killing 'power' is impossible to measure and compare, every shot, every animal, every situation is different, there are no constants. Once you go down this discussion paths everyone has a million different opinions, and we cant prove hardly any of them
Shot placement and bullet construction are MORE important than anything else, no animal will be able to tell you if a bullet was 0.02 of an inch bigger or smaller, or 50fps difference in impact speed.
What you should be looking at is what rifle can you shoot comfortably? Is that enough to get the job done? If not, why?
I have just returned from my first international hunt as a hunter/client(for mid Asian Ibex), I packed my 300wsm which I do not overly like, one other hunter had a 300wsm, and my uncle packed his .243.
Guess what, the Ibex shot with the .243 was every bit as dead as the ones shot with the .300wsms, I missed a couple of shots, the other 300wsm shooting scoped himself badly.
My uncle fired one bullet, killed one Ibex.
The most experienced hunters I know generally shoot with small calibers, even .223s,.222s, on 'big game'.
Im not saying you should go out shooting elk with a .222, but your far better off having a milder caliber that you shoot well, is easy to carry and cheap to run.
You can only kill animals once, I think people have become obsessed with 'dropping' game dead on the spot. A good shoot with an animal running a few yards is no worse than an animal dropping dead on the spot.
Being able to shoot effectively is the number one most important factor to killing game efficiency, its that simple.