yep thats about right.abouttobeglue said:
If you improve the air flow out of the engine it will also want to suck more air in, thus it will also burn more fuel.
Also if there is an increase in sound and throttle response (which there should be) you are much more liekly to drive that bit harder, which will of course drop the the mpg.
If you want performance don't worry economy. If you want economy don't worry about performance. They do not go hand in hand!