I went from two 560 Ti's to a GTX 670. Mostly because I was getting annoyed that the 560 Ti's ran really hot and loud. But I noticed some other subtle differences. For instance, I noticed that in Skyrim I was getting smoother performance (with my setup, it would constantly stutter, with a single GTX 670 it never did). And I think I felt like I was getting smoother performance overall.
So you probably will get some subtle, more pleasant changes like this. GPU performance now isn't always about FPS (especially since multi-GPU setups have this thing called microstuttering)