I have had the pleasure of hearing the Symphony II+ in a non-bose A4 and it sounded much better than my Symphony II with Bose, but that car did not have any satellite radio, so I could not compare.
With that said, I find that my XM sounds much flatter than FM or CD, and imagine that the CAN bus is to blame (at least partially).

Anyway, could someone who has experienced XM or Sirius with the Symphony II+ chime in and tell me what the audio quality is like? I have a line on a Symphony II+, but my main goal is to improve audio quality on XM, which is what I listen to 99% of the time.

The II+ radios need a different antenna system than the II radio, so FM will sound worse if you upgrade. At least in some areas.

also, I dont think its an issue where I live.

It's not a FM thing, CAN is not involved. The SAT receiver/tuner audio out to the HU is analog line level, not a digital signal. The SymII+ sq is superior for all sources, SAT, FM/AM, CD all sound better than the Sym II.

The amount and type of digital signal compression/decompression used for the SAT downlink effects the SAT sq alone, but the SAT will sound better on the Sym II+ regardless.