The OEM HU usually will only talk to the OEM Sirius adapter.
On previous Subaru radios that have the Clarion CeNET expansion port, people have tried aftermarket "generic" Clarion satellite tuners, but they didn't work.
Newer Subaru radios have a proprietary bus port (20-pin).
On the 2011, the stubby antenna unit on the roof may already contain a sat radio antenna. Then you'll have an antenna cable already tucked behind your factory radio.
Earlier years were "pre-wired for satellite radio," which simply meant there was an extension cable that ran from behind the radio to above the map light cluster. That just saved you from running the cable down the A-pillar and under the dash. When you (or your dealer) added the sat radio accessory kit, they would stick the kit's OEM sat radio antenna on the inside of the windshield, at the upper-right corner.
On earlier years, the OEM GPS antenna was installed under the dash, above the instrument cluster. People have placed aftermarket GPS antennas under the dash, above the HU and they've worked fine there.