Even though I'm aware that the Grand Prix Store (GPS) PCM isn't the best choice out there, it does help out some. The main reason I keep using it, versus stock, are the transmission and shift features.
For the record, the Grand Prix Store claims the following features:
With my cam and heads, the raised shift points help out significantly. Although, I could certainly stand another 700 RPM or so. Also, the elimination of the "40 MPH Lag" with use of the skip shift makes the car much faster from a roll.
I can't comment on the effectiveness of the fuel and timing tables, but I do know that people have converted from the GPS PCM to a Digital Horsepower (DHP) PCM have felt rather impressive gains. Take this as you will; I plan to buy a custom DHP in the near future.
One important thing to note is that the GPS PCM is more prone to knock and is more sensitive to bad gas than the other PCMs currently available. Therefore, for people that live in octane-challenged areas, this PCM is certainly NOT a good selection.