From my tests, on the same system running Win 7(with all updates), and win 8.1(all updates) the performance is 3-10 fps lower in 8.1(so close to the margin of error that its not really a consideration for games running at 150 fps)
Guy who was talking about SP1 on win 7, service packs are just large collections of updates made for easyer install, in windows 8 thing were so bad they went for a version number addition instead of releasing a service pack, I had no problems running any of the games in windows 7
bf4 had sound issues untill I did a cfg file tweak in win8.1
NFSW wouldn't play nice with my controller in windows 8.1, had to do a reg edit
games tested at 1080p max settings (Brackets for win8):
BF4: 189(183)max 89 min, 149(144) avg
Hitman absolution: 123(120) max, 73 min, 97(95) avg
NFSW: 219 min, 670(650) max, 312(310) avg
Background o me: Being a builder o custom computers, someone who laughs at stickers that say "no user serviceable parts inside", such as replacing TVS diodes that are blown on hard drives that died to power faults, and etc, also having the ability to remove viruses without the use of a virus scanner, an avid tinker, my router is home built and has more oomf than the average persons computer