if your putting on headers and a full exhaust, your gaining great horsepower, but its going to be louder. But were talking a great deep v8 sound, not anything those small 4 cyclinders can make -those are the "loud" cars. the stock g8 gt has 2 mufflers and a resonator, its dead dead quiet. Any exhaust you put on you'll love. Plus its not like any music in the car instantly covers up any interior sound that gets created with a new full exhaust. My one highly highly recommendation though, it getting a tune to get rid of your DOD (displacement on demand). Once that kicks in with a full exhaust, it creates a really low deep vibration sound, which is kinda annoying. But any full header and cat back system you put on your going to love it. most of the time I drive is with my windows open and listening to the sound of the engine and exhaust cough.
2009 G8 GXP - M6 PBM
Sunroof, Onyx, Roto-Fab, Corsa, Tune, 5k, LED fogs
404 WHP, 404 TQ
2008 GSX-R 750
Full Jardine Exhaust, PCIII, K&N Race Filter
Best 1/4: 10.67