You won't see a dramatic horsepower increase from just a performance exhaust system on a 2006 Pontiac GTO. While you might gain some horsepower, it's typically a relatively modest amount, often in the range of
5-15 horsepower. The exact gain depends heavily on the specific exhaust system (brand, design, materials), the rest of the car's modifications (if any), and even the dyno used to measure the gains.
It's more accurate to say that a performance exhaust system will improve the *feel* of the car more than dramatically increase horsepower numbers. The gains are often more noticeable in terms of improved throttle response, reduced backpressure, and a more aggressive exhaust note. Don't expect a huge jump in power figures.
