The latest 301.24 drivers use the FXAA injector or so. See if you might have enabled it by chance.
21% increase doesn't necessarily be present. That amount of increase might have been on a much higher end card, like the GTX560/570/580.
If you see an FPS drop in it, you may as well roll back if you want to. Also, these are the beta drivers, so it might not be working well for all users.
I'm currently using the 301.24, and I don't see much of a performance gain or loss from my old 267.85. But still, I'm keeping it for a reason.