My factory system could be better, and the path to upgrade from 6 to 10 speaker (BMW includes an amp in the back) involves adding under carpet wiring, front & rear door tweeters, etc.

I upgrade my BMW's to add factory options or similar, so know what & how. My X5 has received PDC, multifunction wheel, rear seat foot well lights, glovebox flashlight socket, etc. Moving from 6 to 10 is a BIG hassle.

I would first recommend you install (I used Dynamat) sound deadening tar / similar in your doors. Makes a better sound when you close them and it makes the car quieter - hence makes your system more effective. Removing door panels is quite easy, figure 30 minutes per door for the upgrade.