Originally Posted by BMWM.D.
The thing that's really kept me from investing in the "is" muffler swap has been the performance gains, or lack of. A full downpipe-back system like HKS, ETS, etc. will give at least 10 rwhp for the effort, along with the sound. Plus you can keep your factory exhaust to go back to stock easily if you ever want to. I'm really loving the tone of the ETS system from the videos I've seen, so I might go this route instead.
Frankly, that too has kept me from the 'IS and PE exhaust, no bang for the buck.