As far as I can tell, the high-end Asus p8p67 boards drops the multiplier to 16 when the cpu is idle. Try running a cpu benchmark program and watch the CPU-Z results. I have 2500k+p8p67 pro combo, and I was getting it to peak at 4.3 ghz under load just using the autotune feature.
I just noticed that a bit ago running some Passmark benchs. It lowers the CPU while idle and then ramps it up while under load. Woot! Now if Asus would update the BIOS on this board it'll be more stable. Ug, lol