Well after changing some settings again and restarting the game every time I changed something. It suddenly runs very smooth again in singleplayer and in some multiplayer missions getting up to 50-80 fps. But sadly enough, the mod I enjoy the most atm is Wasteland and I'm getting max 22 fps there which in alpha was still a lot smoother.
But after viewing this site you gave (http://www.bit-tech.net/hardware/cpus/2012/05/01/intel-core-i5-3570k-cpu-review/6), it's kinda very obvious that the game is COMPLETELY optimized for intel CPU and AMD is COMPLETELY forgotten. A friend of mine has an i5 2500K with +- same videocard and the game runs very smooth on his PC, while i have a phantom 2 955 which is almost identical speedwise.
If you look at the benchmarks a CPU AMD FX-8150 (4.818GHz) scores worse then a 2500K @ 3,3ghz... what a joke :s
Yes i'm aware that the benchmarks are done on Arma 2 but Arma 3 is an upgraded engine of Arma 2, so the results will be the same.