Since Mutter was replaced with Compiz to improve Unity's performance, moving back to Mutter might not be an option.
On the other hand, because of the Plasma Active project, KWin was stripped down for mobile, and a Mer based distro that uses Plasma Active can already be installed on a Nexus 7. Also, work is being done to support Wayland in KWin.
I don't fully understand the reasons behind the Unity strategy, it seems to me that embracing KDE would be a more efficient way to achieve the envisioned "One OS fits all sizes". KDE already has an offer that suits desktops, netbooks, tablets... and is designed to make easy building yet another interface if that's what's needed.
Sure, KDE is resource hungry, but Unity is not a bit lighter, and the KDE devs are improving Plasma's performance with every release.
Well, perhaps this is Canonical's "plan B"? Since 12.04 there's a Plasma Active Kubuntu for i386:
http://cdimage.ubuntu.com/kubuntu-ac...ecise/release/