I always believe that it is the hardware limitation of PS3, GT5 or now GT6 will have 1200 cars, meaning if they use 1200 different car samples, with each have their own effects, different pitch samples, conditions, etc, the sound library will be massive, I'm talking about hundreds of GB.
I'm an amateur composer and uses plugins that uses real samples from real instruments like for example: Trilian.
Trilian is an excellent plugin that can generates bass sound that sounds very real.
here take a look
http://www.spectrasonics.net/products/trilian.php
look, they have only 60 different instruments and the library is a whopping:
and that's only for storage. NOT for playing the instruments real time.
listen to their demo:
now in order to play 12 different car sounds in the same track, with each have their own samples, effects, etc, I don't think PS3 could handle it anymore. I don't think the RAM is sufficient enough, and the processing power, since we know it already consumed by the stunning GT6 graphic.
producing perfect sound is not as easy as recording and replaying it with a click of a button, it requires huge amount of RAM.
that's why not all car sounded good, I think that's because that limitation.