well i had 450s running 1 bar boost and mileage wasn't very bad a little worst than stock.....
but only if i kept the pedal to the floor, on normal driving i would say it was near identical
Mine is fine mate if i dont boost it or really put my foot down the consumption is amazing ! Of course if i open it up then it really starts too guzzle its all relative really If you have a heavy foot then your petrol will be used quicker.