Hi Gavin,
I took another look, and you may be right. I would suggest giving crutchfield a call or send them an email. they are VERY knowledgable about what works. I have been using them for over 15 years.
one thing I can suggest to save you a little hassle, the antenna for the satillite for me works fine on my dashboard. (you can see it in one of my pics) the directions and everyone will tell you it needs to be on the roof. but I just put it on the dashboard and it works fine here in Atlanta.
My friend has that premium ford radio in her mustang and I agree it does sound good.
Anyhow I hope it works for you, but if you decide to go whole hog like me, just let me know if you need any install advice. I have had my dashboard apart 6 times since I got my 07 2 weeks ago.
OOOO I do have one tip if you do install that xm unit. an easy way to get to things is to open the glovebox then press the side tabs in and then the door flops down giving you a nice big hole into the dash.
Make sure you get the dashboard disassembly instructions from crutchfield, it will save you time.
cheers,