I seem to recall that the Steambox OS were supposed to be based on Ubuntu, but since Valve listens to their customers, my bet is that if they still base it on Ubuntu, it won't be with a Mir-based Unity.
Now, if anyone has pull when it comes to drivers, I bet that gaming companies has much more pull than silly small, yet weirdly successful companies (I really struggle to understand Ubuntus success with Unity. All other Ubuntu distros are nicer looking and mostly more stable).
I still wonder if Unbuntu install stats, are all Ubuntu distros, or if it's only Unity-based Ubuntu distros. Because most non-unity based Ubuntu distros will be running using Wayland. In short, if Ubuntu doesn't gain massive success on mobile platforms, they're in trouble.