There will be no performance hit. I moved a 980 Ti from a P67 ( PCI-E 2.0 ) board to Z170X ( PCI-E 3.0 ) and it wasn't noticeable. 980 Ti is roughly equal to a GTX 1070.
Im just really Mod edit: keep it clean please repaired bsod, gpu driver stopping working, and just wanted to.get better gpu after all these years that my radeon burned.. all i want from it is to work with at least 99%of performance :c
As I said there will be no difference. Assuming the rest of your PCI-E 2.0 system is good anyway. I went from a P67/2600K to a Z170X/6700K and saw very little difference in gaming. Just moved the same 980 Ti to the new system. As long as the PCI-E 2.0 system isn't bottlenecked by the CPU you are fine.
Going from a 2600K on P67 to a 6700K on Z170X gave me less than 5 FPS in the games I was playing at the time. The Witcher 3, Fallout 4 and Rise of the Tomb Raider were some of them. 980 Ti and 1440p. There was no bottleneck on PCI-E 2.0.