I've been debating for some time, but I've finally put an Overkill tune on the GXP.
Main reason I was debating is that the car is pretty much stock. An intake, Transgo Shift Kit, and some mufflers. But everything I've read indicated his tune would wake up even the stock. I have to say I'm impressed. The pedal feels more responsive, transmission seems to be more aggressive when getting on the gas. And finally got rid of the bog out that occurs when traction control kicks on.
Everything I've read about the tune has been spot on. I'm sure there's a small part placebo effect as I just got the tune. But it's definitely different and more responsive.