I just helped a buddy install XM into a Honda Accord. It was very easy to hook up. We used a small dot of velcro to affix the tiny antenna to the dashboard. That was a lot easier then following the instructions of mounting on the trunk lid. We simply plugged the cigarette lighter type power connector into power outlet (cigarette bype receptacle), and we were in business. The XM radio output needs to be adjusted to an unused FM frequency in your area. Once adjusted, tune your car FM radio to that frequency to listen to XM. The little XM radio receiver clips onto an A/C vent grill.
I don't know what hook up Subaru has provided for Sirrius radio, but since only electrical power and antenna hooks up are needed, my guess is that is what Subaru has provided.the electrical, maybe the antenna. The electrical you can certainly use, the antenna may be different. Your XM kit will have the auto antenna.
We also used a different kit to hooked up XM in his house, so that it would play thru his home stereo. In the house the XM antenna needs to be pointed to the South.
Hope this info helps. Regards, Larry