Pretty much anything aftermarket is only there to be loud.
I'm going to disagree with that statement.
EVERYTHING I make gains power, at an average of 10-15% (or more) hp and torque. Yes, they're louder, but not until you really twist on the throttle pretty good. Low rpm, off idle to 3500 rpm, what I make is about the same as the factory race exhaust. It's louder above that, but not obnoxiously so.
Got results back on another 2003 XB-9 since this one hadn't been tuned yet, stock is 75 hp, 60 ft/lbs, final was 87 and 70 ft/lbs of torque.