The mpg does not go up. If you are reading the stock mpg gauge, it is no longer accurate because you're running larger injectors.

Does it make sense to put 200lbs of equipment in your car and get a 4mpg increase?

you add a tune which is done perfectly for your motor only= horse power
Cold intake temps due to intercooler= horsepower
turbo spooling even when not in boost mean less restriction in the intake and more air density which means better combustion= horsepower
lightened flywheel= horse power
ALL this added up equal horsepower when you need it and better gas mileage when your driving normally!

o I forgot greddy turbo tic exaust and down pipes and underdrive pulley