My friend, you can't judge a mod if you're IGNORANT about the N54 engine. You need to read more 411. You're questions are answered in this thread.

http://www.e90post.com/forums/showthread.php?t=147399

Check out the Dyno sheets and this one i got from N54tech.com. There's an obvious WHP gain. Also, you can't compare a 1.8 to a 3.5. Two different engines. Obviously, a bigger displacement benefits more on this kind of mod IMO.

FYI, an exhaust for a turbo car is a plus because the stock muffler is restrictive just like the stock air box.