I think the noise energy thing is a little more complex than most people think. The key with cars is flow, its all about getting the gas to flow out of the engine as efficiently as possible and it varies with RPM, and quite a few other things. The noise is not actually harnessed as motive power if you keep it in, it is just absorbed in the mechanics of the exhaust I think, so I am not sure the extra noise necessarily removes power. What normally happens is that the modifications to the exhaust alter the air flow and make the air flow less efficient so the engine is less powerful. if the mods are done with a rolling road and the ability to alter the fuel mix and ignition map, it can be good. However it is generally very difficult to reproduce the instrumentation the manufacturer uses to optimise an engine and its exhaust. I did some work in Lucas Research in the engine test centre and was impressed with what can be done.