Borla exhaust note on 2005

Does anyone know if a Borla exhaust system from the cat back will add any horsepower on my 2005 GT? How much louder will the exhaust note be vs. the stock setup? Seems the only time I can really hear it now is from inside the car with the top up.

Also, do I need to reflash the computer if I add an KN kit? What about replacing the throttle body?

Thanks,

Steve