There are firms (some host this forum) who have upgraded speakers for that car. This is definitely the lowest-cost approach.
The bottleneck is the amplifier and the architecture of the system.
Best results are obtained (NOT lowest-cost results) when replacing the amplifier
and the speakers, and abandoning the BMW front-door three-way architecture and using a two-way front architecture with a tweeter that plays low into the upper midrange.
Here are some links with some descriptions and pics:
http://www.xoutpost.com/bavarian-soun...005-4-8is.html
http://www.xoutpost.com/bavarian-soun...io-system.html