Those silencers have to rob your car of power...I mean, it's basically plugging up your exhaust, doing the exact opposite of everything most of us spend time and money to do (create a more free-breathing engine, from the front of the intake to the very end of the exhaust)...they might say that they increase mid-range torque, but they've gotta KILL high-end HP. I mean, replacing my Invidia with a stock exhaust system might increase mid-range torque, but it's never gonna improve the overall and high-RPM performance.
My car was loud as hell when I first got it. It had the aforementioned exhaust, a homemade test pipe, and a DC Sports header. I ended up replacing (on the "advice" of a local police officer) the lame test pipe with a Magnaflow/Carsound high flow cat, and it made a pretty big difference in the car operating at a reasonable sound level. I wouldn't mind it being quieter at certain RPMs under acceleration, but I won't use a silencer -- I'd rather go see about getting another resonator welded into the system.