Yes, magnums do offers more "aggressive" expansion on contact (all else, except velocity, held constant), however, this is a mixed blessing because reliable killing is a function of expansion and penetration. Thus, it may well be the case that the magnum drops the animal faster with a lung hit, but would be more problematic on a hit where bone and otherwise deeper penetration would be required (again, holding fixed bullet construction).

My also conjecture that, for most guys, the ability to place a bullet is the binding constraint, not ballistics.
Now, to be sure, there are some that have taken the time to practice a lot with their magnums at 300, 400, and 500 yds. And these folks have worked up an super-accurate and powderul load, and have mastered its trajectory and windrift properties (both in steady winds and variable, gusty winds, not to mention the ability to compute the windspeed in the field), and, also important, how point of impact changes when shooting up and down hill.

For me, shooting 300 yds is the farthest I can practice. With target-power scopes and a rock-steady bench I can produce some excellent groups at this range with 6mm BR up through 9.3x62. But when the the 1.5-5x or 2-7x hunting scope goes back on after load development, and I am looking 250-300 yds down a hillside with only a tree to steady my shot, and my heart pounding from excitment, I may still pass-up the shot, but not because of my standard round's lack of ballistics.

Just my thoughts...

I had my 7mmRM rechambered to 7mmDakota as a long range rig. It is very flat shooting, & I can pretty much hit what I want out to 400yds. Trouble is, the farthest I have taken big game w/ it is about 125yds so I could just as well be using my .280!