I had been searching for the answers to a couple questions regarding swapping the HU in my WRX but wasn't able to find anything definite regarding the Factory Sirius and Media Hub. I did the HU swap with a Pioneed Avic-x710bt over the weekend and got my answers.
As I suspected, the media hub has a proprietary connector to the back of the stock clarion unit, so it was unusable with the new setup. The AVIC has USB anyway, so that's not a huge deal for me.
The factory sirius unit's antenna IS usable with the Pioneer (and I assume everyone else's) CD-SB10 and SCC-1 setup for Sirius. The stock clarion Sirius unit obviously wouldn't work for me since I went with a Pioneer HU.
The stock antenna connector has a thick pink plastic housing on it, but if you break that housing off, it'll connect and work with the SCC-1 Sirius Unit. It saves you having to run a new one, at least.
Obviously if I was able to do it again I wouldn't have gotten the media hub or sirius in my car when I bought it. Live and learn, I guess. Hope this saves someone else a couple hundred bucks.