If you OC using ASUS's own OC software then I don't see why would it void your warranty because the GC is meant to be overclocked anyway.

And anyway the GC's own OC settings will definitely not fry the chip cos most prolly if they push also is 10~15% extra only. Unless you OC your own using other software that push it more than that then its different story.

But then again, I doubt they will find out cos OC with software is just set to the OS. You unplug the GC into other PC it will run back at stock speed.