I would like to add to Technick's list of "rules of thumb" that if a tuner can guarantee better fuel mileage usually they are a little too conservative on the tune.
Making global changes to the car is not really the correct way to tune for performance. If you bump the fuel pressure and up the timing you are going to see a performance improvement at WOT but in most cases the idle and PT suffer...some may experience bucking do to
adaptive control.
The largest increase I have seen from a basically stock 5.0 with exhaust and a MAF sensor was 32hp @ 20ft.tq. forced in open loop.
I am currently talking to Superchip's on a chip for my Eclipse GST.