Thanks for the link.
Has anyone done this and can say if the improvement is worth nearly £400?
And how big a job is installing?
from a quick read you have to remove the rear seat!
BMW seem to offer the kit for £400. might call in and see what the cost will be to have them do it.
I have the HiFi option factory fitted to mine, which from my understanding is the same as the Alpine upgrade kit..the tweeters and amp certainly look the same.
I've not heard a base stereo setup, but I love my HiFi. Sound quality is very good and cranks up rather loud before it starts distorting.
For £400 though I'd probably be looking at other possible options such as second hand Gladen kit..there is a guy on e90post selling a full Gladen DSP setup including amp (with bluetooth module for remote configuration), under-seat subs, component speakers, all the wiring harnesses and custom made amp mount etc.
Basically it will give you a rather powerful but completely stealthy upgrade to your base setup and he's asking £550. Installation would probably take you a good couple of days to do it properly, but you have all the hardware required. I was going to grab it myself but it wouldn't be a straightforward install for me as I have an amped setup already.