I spent 30 min today in a 328 with the standard audio. I was hoping that it would be good enough to save me $1,900. I just couldn't see myself not regretting the decision every time I got in the car. The standard system lacks mid-base volume and seems to roll off in the high frequencies. The base itself is fine (if not very crisp or deep but definitely packing some punch) but the lack of linearity across the hearing range is annoying if you know what the music is supposed to sound like. I did not notice any distortion, which is a good thing. In my amateur opinion I'd say the problem is with the crossovers or speakers because I don't believe you can buy an amplifier with that non-linear a response. The amp had enough punch from my perspective but I'm 40 and don't listen to music that loud anymore. With that said, the weaknesses were much less noticeable with heavy metal music so if that is your cup of tea then perhaps you can happily enjoy this system. I found the standard system to be not as good as the upgraded audio in my 97 E36 M3. I was really disappointed, but glad I took the time to listen for myself as the standard audio would have been a disappointment. You might be able to "fix" this system with after market speakers if you really can't afford the premium system but with trial and error you could quickly find yourself at the $1000 point anyway...
Note: The standard system has two front speakers in each door (I believe a tweeter up high opposite the mirror and a mid-range lower). I believe there were sub woofers under each front seat. There were not speakers in the rear doors (this was a sedan). I assume there were four more speakers in the rear deck to get the total of 10.
I listened to the "premium" system for another 15-20 minutes and it seems to sound acceptable for a car of this quality. The mid bass region (100-500 Hz is what I'm talking about) seemed much stronger and crisp and the high frequencies were also louder. The bass response seemed improved but I was happy with the standard system's bass response. The amp puts out way more volume than I'll use but I would enjoy hearing good music (not my heavy metal crap) on that system.
IMO, neither system is really a high end system but the premium system sounds as good as anything I've heard in a car (not that I run in circles where I hear nice car stereos often). I could live with the standard system but I think I would notice it every day. As I sat in the car and contemplated ownership it seemed silly to spend nearly $70K on the car and then be irritated by a sound system that doesn't sound as good as my 11 year old car just to save $1900. Kudos to anyone to whom the standard system sounds acceptable. I wish I had your ears, I'd be able to save $1,900!