All I can tell you is either what I've seen or what I've read. Based on those two things I go with the thought that is is not just energy. Consider that at some point a 50 gr. Vmax fired from a 22-250 has more energy than a 240 gr. bullet fired from a 44 Mag. Obviously one would be more appropriate to hunt elk with than the other.

And 1000 ft. lb. of energy applied in the correct place would be far more effective than 3000 ft. lb. poorly placed.

The common accepted thought being it should require 1500 ft. lb. to be suitable for elk. For what its worth.................