I have been looking to buy the P55 FTW for some time now so this research is pretty fresh in my head.
Tiger Direct lists the manufacturer specs as:
Memory Supported: 1066MHz DDR3
I am also fairly certain that with the editing features available in the bios that you can push the RAM clock speeds to that 2000+ that you were referring to, but I wouldn't try that with any old RAM. Best of luck.