It seems that the vast majority of games do not differ at all in terms of performance between the two operating systems, and if they do, its usually pretty negligible (1 or 2 fps either side). Early issues appear to have been ironed out by this point.
Some people report FPS increases, some decreases but nothing significant. I for one havent noticed any real differences after going from Win7 to Win10, then back to 7 and back to 10 again a few months later.
The one important thing is DX12 support. While there arent any mainstream games that run on DX12 right now, it will become an important factor in the near future as it allows developers to get more out of your hardware than previous version did.