Played a few titles on my Windows 8 HTPC and honestly can't see any difference in performance. If anything Windows 7 boots up faster. As above, if you ditch the metro tiles they're pretty much the same from a performance and visual perspective.
My experience is the opposite,infact just did a clean install of Win8.1 RTM a few days ago,installation was fast ie 10 to 15 mins,bootup is very quick and yes I was using Win8 since last October and Win7 before that so Win8/8.1 is snappier to me then Win7.
Gaming wise its all relative down to your hardware and drivers,but Win8/8.1 does have better DX support, improved security so all things gamers should look for.
Oldest game I've tried is XCOM- apocalypse(1997 game) so that is even pre Win98 which works fine using D-Fend software for DOS and very old games,no issues with classics like Baldurs Gate,Icewind Dale series ,KOTOR etc...
So no issues so far in any of my games,as to anti-virus well I'm trying out AVG 2014 Internet security and that is working well with my Win8.1 Pro x64 RTM,end of the day AV is a preference thing,some are happy with just Defender,I personally run a malwarebytes scan now and then(free version) to be a bit on the safe side.
I did read Win8.1 has improved security over Win8 (which itself is improved over Win7 security wise)so that is always nice to have.
Side note Win8/8.1 is still easy to navigate and I don't see the big issue still with some people moaning about bringing back the old classic start button,its not needed IMHO and Win8/8.1 is very easy to customise in both metro and old desktop that is if you can be bothered and accept changes that a new OS brings.
I don't use touch still a mouse and keyboard desktop/gamer user here for Windows.