I've been using an bluetooth 3.5mm adaptor I purchased from ebay and it's hard to say, simply because I made adjustments on PowerAMP, using the equalizer, so the sound is still pretty darn good...and still seems to transmit in stereo (possibly because it's still connected to the 3.5mm adaptor vs. going direct bluetooth to the headunit).
That said, there was a brief moment where the USB connecter was not allowing the unit to charge and I had to use direct 3.5mm input. When I did that, I did notice the sound jumping up significantly from bluetooth. Again, not an issue for me because I can make the adjustments through PowerAMP's equalizer, but it might not be acceptable for many people.
Were there no equalizer settings on the OEM headunits (on the cars you test drove) that would allow for Stereo bluetooth transmission via A2DP?
Love the G-Note series!