I have a 2009 128i vert. The 2008 and 2009 models in the U. S. had the mid level system for the rest of the world. It has 4 tweeters, 4 4" mids and two 8 inch subs under the front seats. The tweeters for the front are by the side mirrors and the 4 inch are in the doors. On my vert, the back speakers are on the sides but in a coupe I think they may be up by the rear window.
I would call the sound good but not great. The quality of the speakers is better in the top level system and it also has more power. I don't listen that loud so my tendency is to believe better speakers would make the most difference. Others argue more power on the current ones helps a lot. One way to go if you agree with my quality of the speakers assessment is new speakers from Bavarian Sound Werks. Some will argue they are not that good, other report the opposite.