Um, what makes you so sure that such a setting exists in the first place? It's not like developers sit around thinking of new ways to drain performance without increasing image quality. Anyway, if you need more FPS, the most dramatic changes you can make are to make sure MSAA is disabled, keep ambient occlusion at any setting other than "HDAO," keep your "PostFX" quality setting at "medium" or lower, or keep your "shadows" quality setting at "high" or lower. These changes will all definitely have a "discernibleo impact" on image quality, however.
I know people mentioned lowering Postfx and didn't seem to notice much of a difference in quality. I just wanted to know if there were other settings that could be dropped or enabled through gpu for performance gains. I realize there is going to be some quality trade off , but sometimes it's not worth the performance hit.