I quit using my 7MM RM for most of my deer hunting! It's not that the 7MM mags are not good for deer, it is excellent! With some overkill! Not that I am against overkill.
I feel that the problem with the 7MM mags on game less than 200 lbs. is the bullet, not the speed or the caliber! Most bullets are designed to expand best at certain velocities and ranges. Most bullets designed for magnums are designed for heavy game, at longer ranges. When you hit a 100-200 lb. whitetail at < 100 yards, the heavy constructed bullet may not expand much, if at all, at any velocity! Especially if your not hitting much bone, such as a lung shot!
As for the myth of "knockdown power," ANY decent caliber, even with FMJ bullets will knock down a deer with head, spine or double shoulder shots! That's just bullet placement!
So, if you are going for double lung shots on smaller game, just be sure NOT to use a bullet designed for a 600+ lb. animal! Or you will likely have an animal that will run anywhere from DRT to 100+ yards, with little or no blood trail to follow!