Mine is chipped with no other mods. I estimate a gain of around 5-8% (12-18BHP) in power, but the car is much more drivable across the range and picks up more quickly.

The chip lets me go into the redline before the engine cuts out the petrol. I can drive in 6th (manual) from about 22mph around 800rpm without stuttering. My fuel economy is also much better.