Most definitely you will benefit from shooting a gun fitted properly, all else being equal (proper mount,etc.).

I proved it to myself recently making a stock for myself. Starting out with the comb a little high and fat, I took it to the patterning board. It was interesting to watch the pattern move across until it was where it needed to be.

I make no secret of being able to shoot a gun well enough to adapt to one that isn't right. Anyone who's shot with me will attest to that.....