I have 2x GTX 760's in SLI and i havent had any FPS issues so i wouldnt say for sure that its an Nvidia problem. I also have a pc the is running 2x GTX 460's in SLI and that one runs great too. 460's are running at 1920x1080 and the 760's at 2560x1080.

Edit: Both on max settings btw.