There are calibers with greater kinetic energy, and calibers with less. In the majority of cases a well placed shot taken with any legal big game caliber, at reasonable distances, will cleanly kill an elk. Larger bullets, and more energy can provide a greater margin of error if the shot placement is not ideal or distances are greater.
I would not hesitate to use a 7mm-08 on elk at reasonable distances. My son has one and has cleanly killed numerous animals, some as large as elk.