Yes, I do because that's what I'm doing.
I have had my G8 GT completely Pedderized. Every single Black OEM bushing has been removed and replaced with Pedders Red Polyeurethane, front and back. I'm saving up money to replace the stock oil and air shocks with Coil Overs which are far superior and give the car a feel as if its on rails. Then thats all you can do to increase the handling, ride, suspension and control of the car. Pedders Suspension - Pedders Suspension - More than just great products... No Bull
Next I bought a roto-Fab from the pontiac dealer to increase the horsepower. Your car will really come to life after a Roto-Fab, and save a few ticks on the gas mileage too. Roto-Fab
The only other modification you can perform without voiding the manufacturer's warranty and any extended warranty is a performance exhaust system. I'm going to buy a Corsa Performance Exhaust system the Crusing, not Sport style. That will increase horsepower another 15 - 30 depending on how the car reacts. Corsa Performance Exhaust
If you tune your car, you void the warranty. Most people I know doing those kinds of mods have equipment to untune it before taking it back to the dealer for any repairs. But that is such a hassle, just leave it stock, don't tune it, or make any modifications that alter the interior of the engine and your fine.