I have the BMW Performance exhaust on my 130i and I'm very pleased with it. It has just the right amount of noise when on it and also still quiet when using lighter throttle openings with no boominess in motorways.
Do note that it's only a back box and not a full cat-back system.
Here's my review here with pics -
http://babybmw.net/forum/viewtopic.php?f=17&t=11769
Other systems available include Baystuck and Supersprint although I've not heard these systems.
PS have you enquired at the dealer with the correct part number for the exhaust. Maybe you can get it?