I had once a 3570k that was limited at 2500Mhz by IMC, but at leat it did 2400, 100% stable.
You should try the cpu+mem on another motheboard , Asus recomended to see what happens.
Z77 Extreme6 has validation on hwbot at 2700+Mhz dual channel so it's hard to say if the motheboard it's defect.
You have some 10-15% chances to have a very poor cpu on IMC.

Have you tryed one module, single channel? Any difference?