About the only thing that I learned I could always count on when elk hunting was "Nothing". There are a lot of "probablys", but not too many "definitelys". About the time you think you have them figured out, elk will do something totally unexpected.
You can "probably" kill an elk with most any caliber. We have all heard the story of the then world record grizzly bear being killed by an Eskimo woman with a .22 rifle, while picking berries. However, that would not be my caliber of choice when facing down a charging grizzly. There is a big difference in shooting a calm undisturbed, animal on a private ranch and shooting one that has been chased over half the mountain by orange clad flatlanders.
The 6.5 x 55 Swede is an excellent cartridge, and is usually the go to caliber for Moose in its home land. I do not know if a Colorado Elk is tougher than a Swedish Moose, but sometimes it sure seems that way. Just another case of picking the gun/caliber to fit the game and hunting conditions.
There is no absolute right, or wrong, answer that fits all game, all hunters, and all locales. My choice is still the big gun with a greater margin of error.