The Harmon Kardon "upgrade" isn't very good either.
It works, though.
Depends on what you're looking for, too. A car is a terrible acoustic environment -- you sit off center, there's road noise, etc. So for example a $2,000
home stereo would sound ten times better than a $2,000
car stereo.
Years ago I had amps and crossovers, etc, in the car (all completely hidden from view, and looking fully stock), but it was more trouble than it was worth, in the end, and I wouldn't do it again.
Today I just live with the Harmon Kardon setup, and try not to listen too closely.
But if you're anxious, you may want to have a look at these kits, which are high efficiency speakers designed to work well with the BMW factory amplifiers:
http://www.bavariansoundwerks.com/category/174/0/BMW-Speaker-Packages/
All you have to do is swap out the speakers.
Hope this helps.