My 2.0l has the HK upgrade and an aftermarket headunit with additional rear speakers and that works well.
My //M, in the picture, had the normal speakers - odd as it is one of the very first ever //M versions and was originally bought by the head of BMW UK and therefore you expect it would have had everything (maybe HK came later?). However the additional speakers in this aftermarket config mean that you can have the top down, AND your foot down AND listen to tunes....
IMO music and driving (or riding my motorbike) go hand in hand. Play it loud when you are pushing the car, AND/OR have it turned down when you have a passenger and are talking but this setup allows the low volumes to still have the depth that the music was recorded with.