...IMO a car is not the best place to be listening to music. I am an audiophile nut and have spent the price of my Australia bound M-sport on my home stereo (2 channel) where I can listen to music uninterupted.
In the M-Sport I avoided the upgrade to the HK figuring the "HiFi" in the 328 was fine for listening within the confines of the car and other external noise. I havent heard the basic system that is in the 318 or 320 but I would agree with statements above that a move from basic to mid will give the most appreciable result for most.
Also....most people are listening to relatively low sample rate/bit depth MP3 in the car
these days and lose much of the audio information that a better spec'd system can reproduce. Like all things it comes down to what you find important but the mid option should do just fine in the car.... Mine arrives in port on 15 Oct so not too long to wait now....probably first week of November for delivery.