Hi,
If it is the same as the E90 then slim
If you want to add tweeters then you cant as you will need to code the headunit, radio from basic to hifi and then you will loose your bass and treble and need an amplifier. If you want to upgrade the radio by adding a amplifier, then good luck (I have a few amps and also want to do this) as you will again recode your radio to hifi to get all the frequencies available and control that by outputs from the amplifiers.
So here are your options as far as I could determine (mine being worse as I have idrive

)
1. Add tweeters - require recode to hi-fi and amplifier (price R7K from BMW)
2. Add amplifier - require recode to hi-fi and have to get high to low converter for connection from speaker wires to converter to rca for your amplifier (or offer up a leg and get the R7K alpine amp and upgrade from bmw)
Please note that for the R7k, your subs are not changed at all, they replace the door speakers and lucky you get 2 additional tweeters (wtf, where is my additional bass as this is what I short)
Option
3. You get the Alpine unit and then replace the subs with the Rockford fosgate 8" or SWS 8" earthquake subs, just remember that it will cost you R3.5K each, you have 2 of these.
So you are looking at between R7K and R15K for a proper upgrade from what you have now.
Sad ne :blueCry:
Check out this thread
http://www.bmwfanatics.co.za/showthread.php?tid=33300