Took a test drive today and spent some time listening to the premium sound. This was the Red Signature Performance at Santana Row.
My take on the system is that it's very good. It's got an accurate clean natural sound. Pretty revealing. Acoustic strings and female vocals sounded great. High's sounded good. The bass seemed good and accurate. Not boom-boom loud but controlled and at about the right volume. With the default EQ, you don't get the "surrounded in a field of sound" feel. The soundfield is thrown more forward with a fair amount of imaging spread across the windshield with some enveloping.
Test tracks included Annie Lenox (Cold, Little Bird from Diva), IGY (Donald Fagen), U2 (Mysterious Way, The Fly), Norah Jones (Cold Cold Heart) and Crooked Still (Darling Corey).
My overall take on the upgrade is that you could do better in a car. But you'd have to pay a lot more than $950 to get it.
FWIW, my home system is Thiel CS 2.3 speakers, the best Velodyne subwoofer I could get at the time and an Integra 10.1 receiver. Total cost probably well over $5K and maybe approaching $10K. It certainly was over $10k by the time I added the center speaker and the rears for the surround sound
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[Additonal note: source material was WAV files ripped from CDs. Sadly the car couldn't read Apple Lossless. Hopefully they'll get that fixed in the future.]