You could play around with the foam in the sound generator to get a sportier noise from your existing setup. Assuming the 2.5l has the sound generator - I'm not sure and I only know what I've read about this as my car doesn't have a SG. See here for info anyway:-
http://www.z4-forum.com/forum/viewtopic.php?f=2&t=4926
I employed a GruppeM induction kit to achieve a much improved noise from my car. :driving: