Over de stereo was ik nog aan het zoeken, staat hier mooie info.
https://musicarnw.com/bmw-e9x-audio-systems/
De iDrive auto's hebben wel een iets sterkere amp ingebouwd zitten dan de Professional CD versies dus het proberen waard met een speakersetje.
Neem aan identiek met de E90 familie.

"These speakers are powered by a 4-channel amplifier IC inside the head unit. The iDrive head unit has slightly more power than the Professional CD player, but both are limited in performance and can be thought of as having similar output as an aftermarket CD player.
To compensate for the lack of tweeters and of larger 8” subwoofers, the head unit front speaker signal is heavily boosted in the bass and treble. This equalization curve is fixed at all volume settings.
The front 4” door speakers and the underseat 6” speakers are both connected to the front channels of the head unit output in parallel. The front 4″ speakers have a series capacitor for high-pass filtering in at least some vehicles. The 4” rear speakers are connected to the rear output channels of the head unit, and that signal is crossover-filtered within the head unit with a roughly 150 Hz high-pass filter"

Dus als je dan die 16mm subs vervangt voor 20cm komen de basjes er sowieso wel lekker uit denk.