I think it has something to do with the sound, not the performance of the enginge. If you upgrade your exhaust with any kit the engines performance will be better aslong as you have upgraded your intake. My brother drives a 87 Nissan 200 SX and he told me that it would sound funny on a BMW if you put a Japaneses Import style exhaust. I would have to agree with him. i have heard a few M3's run at the local dragstrip and the ones that just put rice exhaust on them sounded horrible.
Silver 2000 323i with sport package