I'm in the same boat as you. Mine's a 2010 base WRX, only options were Sirius and Media hub.
I'm in the final stages of putting together my HU swap, a Pioneer Avic-x710bt. I've researched the whole thing a ton.
Unfortunately it looks like we're screwed on the Sirius. Our HU is an OEM Clarion. I've looked at all the diagrams from subaru. Even if you put in a better clarion, it doesn't look like the Sirius connector is the same. So you're probably out the $300 or so for the unit you put in as well as another $150 for a receiver + adapter specific to the HU you replace it with.
As for the media hub, we may get lucky. I have to assume the connector is a standard USB in the back of the HU, so we may be able to just hook the new USB cable that comes with the new HU to the existing cable.
I'm hoping to get my install done in the next month. I've had a heck of a time trying to find the info on all this for our 2010's, so I'll try to take a bunch of pics and post my experience.
Let me know if you find any info to contradict mine.