Can you post pics of your car? Is your tranny the stock 4L60/5-E or some aftermarket tranny. If it's the stock tranny why would you want an aftermarket shifter? Just leave it in 3 and floor it. I would not suggest to manually shift a stock tranny. But I have not seen any GTO specific automatic aftermarket shifters, only on GTO's with aftermarket TH400's. I would think any aftermarket shifter would work as long as it is designed for a 4L60. I know its your car it just my opinion.

If you want more performance out of your auto tranny have your car tuned or get a hand held programmer to get firmer shifts, by geting rid of the Torque Management.