I picked up my P85D with UHF on Wednesday and when I first got in an listened to music over bluetooth I was completely unimpressed and so was my audiophile friend that was with me. I haven't played any FLAC yet so I don't know what improvement I'll see.
However, after several hours of listening to music and making a few adjustments both on my phone and to the equalizer, bluetooth audio was
significantly better. When I mentioned this to my friend today he did point out that speakers need to be broken in some (he experienced this with his high end home speakers) and once they are they tend to sound a lot better. So far that aligns with my experience since the sound has gotten better even after making adjustments and tonight when I was sitting at a supercharger, I was really enjoying what I was hearing both from Slacker and my phone.
As I said, when I first got it, I wasn't happy with the tone, but I did notice that the vocals on several songs were significantly better than my HK audio in my E350. The overall audio tone in the E350 was way better, but the vocals in the MS were clearer and I felt like there was more range and I could hear more subtle notes. Now that the bass has improved the overall tone sounds better, but the bass isn't as good as my '09 E350.
Something to keep in mind is that if you don't get UHF you are only getting 6 speakers instead of 12 and the front door speakers are 6" instead of 7.9" (
based on this). So even if the audio isn't what you would expect for $2500 you are getting an extra 6 speakers that can be upgraded. Those speakers are also already integrated into the car so no need to customize anything. So an upgraded UHF system should be way better than an upgraded standard set up.