AS far as I can see the game ratings are determined a great deal by defence, then rebounds, then everything else.
if you rotate players in defense, in makes just a small bit difference that it at least my experience. So a center you described, playing Guard and defeniding in Backcourt, would maybe make a half till one Rating point more then when he play and defend on guard.
So i am not so sure about the influence on D, because that wasn't an bug which i thought first. And if the multiplier arguments comes, even on SF where C usually get ok Rating(mostly IS?) it don't matters more.