It depends on the cam. The more overlap you've got, the worse fuel economy you're going to get. Plus if it's an auto, it's probably got a converter. That won't help things any either.
If you're sure you have no exhaust leaks (especially upstream from the O2's), you're probably not going to gain a lot from the tune.
05 Quicksilver Twin Turbo GTO
08 GMC 2500 HD Onyx Black Twin Turbo LMM Reading spark plugs