I've got a 780 Ti on PCI gen 2.0 and I'm not seeing any significant hit. My bios is f***** (missing overclocking settings - mainly the multiplier) after an update so I have my CPU at stock until I find a resolution. I've tried flashing an older BIOS version without success.
At stock CPU settings I'm not getting the max performance from my 780 Ti as my results are 5 - 10 fps off what the reviews show. Not a huge problem as I'm getting 60fps+ on all of the games I play at max settings. I believe that once I've resolved my BIOS issue and I overclock my CPU I should gain back some of the lost FPS.