It's too windy to shoot so I'll give this a go...
7mm Mag is a belted case. The cartridge headspaces on the belt not the shoulder. This remains true "if" one is pushing the shoulder back, however slightly, to maintain close to the original distance from case head to shoulder.
I suspect that resizing the cases is pretty much effortless?
However if you back the sizing die off a few thousandths what you expect to happen will happen. The cartridge will shift to head spacing off of the shoulder as with non belted cartridges.
you are satisfied with velocity and accuracy that you are now getting you are in tall cotton. No reason to change anything. Your case life will be good and your powder stocks will remain better.
I'll bet that if you increase the powder charge a bit you'll feel more resistance when resizing then case life will go down due to work hardening and brass flow as pressures go up.
My saying has been 'more powder is better than less powder' and I have a bucket full of worn out cases to prove that's a dumb (and expensive) way to think.