Written correspondance from Superchips -
'The fuel economy on our Ed30 has not deteriorated on day-to-day driving, if anything it is marginally better as you have more torque so the engine is much more flexible. If you use all 300bhp then yes you will suffer on the economy as any 300bhp car but driven sensibly I would not expect the mpg to get any worse.
However, we can tailor the program that goes into your car to suit your needs so if you’d rather have 270bhp (ish) then we can certainly do that for you, it is not a problem. However, having done the development on this car, once you’ve tried it with 300bhp+ you’ll always want 300bhp+!