i5-3570k generally performs better than the FX-8350 in games. Sometimes it by a decent 9+ FPS, other times it may only be 1 FPS, and in games that do not care how fast the CPU is as long as it does not limit the GPU it would be 0 FPS. In general, it depends on exactly which game you are trying to compare.
Programming games to make effective use of more than 4 cores is easier said than done and the vast majority of games only use 2 cores. There are games that can use 4 cores, but again, it depends on exactly which game you are talking about.