I'll have to give your method a try, because supersampling always butchers my performance. Even 2x2 Supersampling via Nvidia Inspector, which should be 3840x2160, is unplayable, and I'm on a GTX 570.
I think that some compatibility bits are better than others in terms of performance (ex. 0x004030C1 vs 0x000010C1 for Kingdoms of Amalur: Reckoning). That's one of the benefits of forcing OGSSAA via EDID override; just go into the options menu and select any resolution that's higher than the display's native resolution.