I haven't seen any hard numbers on what exactly it takes to kill an elk, plus I haven't seen any ML bullets bounce off an elk that have less than 1500 ft-lbs of energy usually by 50 yards. Plus how many handgun hunters have put away their single action hunting pistols that don't even generate that kind of energy at the muzzle. Do I think energy is important, yes to a certain extent. However, I don't think it is a good indicator to how well the cartridge is going to kill.
I first read of 1800 FPS for adequate expansion for Barnes bullets, for a mono metal I guess this might be a good number. I also see where someone posted Berger recommends this as well. I have to ask again what is adequate expansion, and will less than adequate expansion still not kill an elk? Is that inadequate expansion still going to be larger than some other smaller caliber bullets that carry energy and velocity numbers better beyond that of a .30 caliber.
So which one does a guy go by? If you go with 1800 fps then a 150 grain Accubond is basically a 700 yard bullet, if you go by ft-lbs of energy then it's only a 450 yard bullet. If you use the 215 Berger because you can make it carries both numbers to 800 yards, but you have a 400" bull at 875 yards do you pass on the shot if you have high confidence you can make it?
So we can mentally stroke our Johnsons all day over paper numbers for fun, and come up with multiple different answers depending on what bullet is used. I think on paper ballistics are a good place to start, but they aren't the final answer to anything. It's my opinion the answer of how far a .30-06 can be effective on elk can only be determined by the nut behind the trigger, and their confidence in their ability to reading the situation and environment at the time the shot on an elk or other game animal needs to be taken.