@maZiix My CPU is an e6300. Stock speed is 1.86 so I already have a significant overclock on it - using the AC Freeze Pro 7

@kwikfingerz My GTX 460 is also clocked to 850mhz/1950mhz. I did an experiment yesterday - setting the max fps to 40 and monitoring the GPU load with GPUz it barely went above 65%.
@Cinerir looking at what people have posted here and elsewhere, the performance benefit you're seeing is likely due to your Quad.
My performance is pretty good at the moment. 90% of the time I'm over 30fps. Only Orbital strikes and destruction seem to hurt my CPU. My mobo will take a quad cpu so I wondered if a fairly cheap upgrade could raise my minimum fps past 30. A q6600 g0 clocked to around 3-3.2ghz. This would only cost me £60. I think it looks like a worthwhile upgrade.
Thanks for the contributions.