Ubuntu is focused on delivering a great desktop. I'm not a fan of Unity and find that to get the same performance I used to have, I have to run my games on Unity 2D. Regular Unity (3D) eats a lot of my games's FPS.
If Steam picks Ubuntu then I hope they work with Canonical to get Unity to perform as fast for games as Unity 2D (and Gnome 2).
I'd imagine that it would be some serious work to see whether just turning off compositing would be enough and what effects that has on running non-game apps that are in the middle of using compositing.
If Steam picks Ubuntu then I hope they work with Canonical to get Unity to perform as fast for games as Unity 2D (and Gnome 2).