I had the trophy prior to the 8x as well. a few comments on my Bluetooth experience in my mazda 3 2011.
the trophy always had a bitrate issue (which could be fixed in registry to crank up the quality) and all music going through Bluetooth sounded like a very low grade mp3. once mango came out instead of seeing BT AUDIO on my in car display it would then actually show artist/album/song title etc. all stearing wheel controls worked fine when paired.
with the 8x paired to same car the audio bitrate issue is no longer an issue and the sound is about at par with AUX cable, so that's great. i'm not sure what bt stack wp8 or the 8x is using but I do no longer see the info on the display so it just says BT AUDIO, which is a step backward it seems. I believe my car uses a2dp. sometimes my trophy would not pair BOTH phone and music, just one or the other and a simple Bluetooth off/on on phone would resolve it. I have not had this issue with the 8x and it does seem to detect/pair quicker than the trophy did.
incoming calls and messages read aloud switches seamlessly when music is playing on both phones and no issue there.