If I shoot a deer sized critter with a 300 grain bullet that produces 6000 lbs of energy and it enters the rib cage at 3000 fps and exits at 2500 fps I have only used around 500 lbs of energy in the animal. This is when energy becomes largely irrelevant because while it is there it isn't used inside the animal. Large caliber, heavy bullets tend to do just that IME.
Your general idea is right, but the math isn't.
That 500 fps velocity loss does not equal 500 ft-lb of energy - it's more like 1,800 ft-lb. (3000 fps =~6,000 ft-lb, 2500 fps =~ 4200 ft-lb.)
That's more than enough power to kill any deer if the work is done in the right place, and a good example why "overkill" doesn't necessarily kill better than a smaller cartridge.
