There is no bigger waste of money than an aftermarket CAT-back exhaust system. You pick up mininmal hp for making your car infinitely less liveable. All aftermarket exhausts are a scam. They're selling a "sound" not real HP gain. The only way to make a true performance system is start with headers and go all the way back. Remember, internal combustion engines are basically big air pumps. To just change the "back end" doesn't do much. It will make the car louder, though. That you'll notice.
Think about it. Of the total time you spend in the driver's seat -- commuting, driving to beach, track day, etc. -- how much of that time is spent taking advantage of the extra 3 hp you get with that $400 exhaust system? Ah, not much. Now imagine the remainder of that time -- the other 99 percent -- listening to an obnoxious roar, telling yourself how cool your 318 is as you push your earplugs deeper into your ears, your date saying to herself, "nice guy, annoying car..." Yes, BMW engine sound cool -- at the racetrack.
Not worth it.
Gary