Over the near three years I've had my IS I added a Intake, HKS, Header, and S-AFC. It's been fucking LOUD to the point where I finally couldn't take it. I took off the intake and HKS Muffler, put the stock stuff back in and have never been happier. The (near) stock form of this car is just sweet. The exhaust note is beautiful and I love the fact that you can barely hear the engine period, except for at WOT when you want to hear it. I really can't notice any difference at all in performance except for maybe in the shifts. Is that because of the new back pressure??? I think that at sea-level the bolt on mods are a pretty good investment, but where I live up here at 6000ft, the thin, cold air just makes the car way too damn loud and not near enough power to match it. To me it just kind of sucks to have the loudest freakin car in town and be running in the 16s (no it's not my driving, the alt. really does fuck your car up that bad).