Don't worry about the mileage, as long as the engine still has good compression and you look after it well then you can keep on boosting

A G-Tec is a great chip and should see you 25-30bhp extra. You can get a little more power i.e. 10bhp from a custom remap, but it's not much as the turbo simply won't give you any more on a standard car.
For the vast majority of people the G-Tec really works great out of the box

If not then a custom remap simply makes sure that your car is healthy, and corrects the boost, timing and fueling if needed.