Im dead serious when I say inside the car is not bad at all. Of course during WOT it is VERY loud. During cruise it honestly is not that much louder then when it was just the cat-back, more different than loud.
Do your research on the headers if you plan on doing the install yourself. I have read many thread saying that Kooks are the most difficult to install because of the tube design and getting them into the engine bay, same for LG.