Steam OS will just be Steam Big Picture, which is already available. Yet XBMC lives on. And unless you all of a sudden can watch films via Steam, I don't even get what you're basing your ideas on.
You can watch films via steam, at least one film - Indie game the movie is distributed via steam. That said XBMC is so much more.
BTW - I've no idea how good Mir is, it might be the best piece of tech in the world, but news like this make me regret that Ubuntu abandoned Wayland, simply a wide adoption across different projects is important...
It probably won't, Raspbmc renders XBMC using the framebuffer instead of Xorg so it's already quite light on resources, I doubt there would be much that Wayland could improve on that in terms of performance.
That said, this does make Wayland more attractive on other Raspberry Pi distros as a potential windowing system since it's lighter than X11 and is accelerated on the Pi.