I have played in both win 7 64 bit and 8.1 i have seen no difference in performance and i am very happy using the win 7 64 bit .
Im happy with Win 7 i never use Win 8 before is the reason i dont have opinion about it.
Some ppl tell me Win 8.1 is better for games because already have Dx 11.2 (my graphic card support that Dx).
they are very similar in game performance, right? I think still using win 7 x64 Ultimate lol.