SiriusXM compresses the data it sends from the satellites to our car. They constantly change the bit-rate ( 32-64kbps AAC-HE) of each channel depending on the content type, time of day, and what else is going on (for high profile events they turn a bunch of channels down to give another channel higher bit-rates).
They also have terrestrial repeaters in different locations, which are listed here:
http://www.dogstarradio.com/sirius_map.php - these may influence signal quality as well, not sure how that technology works in detail.
The comedy talk radio stuff is heavily compressed and sounds like garbage almost all of the time (muffled and noisy). Some music channels sound great, others more like "meh". Apparently the Howard Stern stuff usually gets full bandwidth.
High bit rate compressed audio on an iPod, memory stick, phone or from CD will normally sound better than what SiriusXM can deliver.
Whenever they add more channels the bandwidth on other channels needs to be reduced, since the total bandwidth of the system doesn't increase. I don't think the system in the Genesis is to blame for the sound quality on SiriusXM data, that's most likely based on the signals received from the satellites ...