I would buy the Sirius module from Vaheh at Crevier BMW. Best price I've found, and GREAT service to boot. Really nice guy and knows what you need. Just be willing to play phone tag for a while : )

I don't know anything about XM Direct. I suspect that Blitz emulates the CD Changer as well (since CD changers are their bag, baby) but I'd ask Blitzsafe.