Had the same issue on mine, exact mobo. I tweaked some power savings stuff and it let me go more. Running 4.6 with a cheap arctic pro rev 2 @ 1.28 vcore, I'm happier then a fox in a hen house. Going to order a SSD today, tired of waiting for the OCZ 3.
I am having almost the same issue except just in the OS. I am running this board with the 2600k processor and the bios reflects a multiplier of 50, but the CPU using control center of cpuid will show a max multiplier of 43. I have flashed using many bios versions, but the same issue exists.