Be sure to try using the 11.6 catalyst drivers, they allow clock speeds outside of the displayed ranges.
If you let ubuntu install the ATI driver automatically, it's a bit of a mess to get the generic 11.6 driver to install.
About all I can offer on the topic was that even though I couldnt ever get the 11.6 to install properly, by the time I was done and went back to the driver installed by ubuntu, I was able to get bigger overclocks.