Montr, Thank you for such an informative post. I've been following all the board posts very closely, as I too have a 328i (just bought it for my wife) and need to upgrade the base stereo...I am currently leaning towards the alpine setup.
I'n no expert, but it certainly would seem that the audio system is much improved by the Alpine system in combination with the head unit tone settings, as evidenced by the 'flatenned' spectrum.
Independent of the measurements....How does it actually sound? Understandable, the volume levels would be much better, but its it worth the price of upgrade in your opinion, or is it just a more expensive tin can?