Huh, guess you got a bad one because most everyone that I've heard testing them has gotten to 4.0 pretty easily.
By that 4.0 I meant 940. I've seen 3.8 and 4.0 for 920 as well from high htt overclock, so I'm assuming they have pretty close air overclockability.

How high are you getting the htt/nb with your overclock?
Or are you just multiplier overclocking?