Yeah, mono 2.7 has an experimental garbage collector that is supposedly wicked good. But, it has to be enabled by a compiler flag specifically, as it's still considered experimental (as is mono 2.7 in general). The latest stable version of mono is 2.6, to my knowledge.
More to the point, we don't interact directly with mono in any fashion -- it's compiled into the Unity 3D engine. So even if mono 2.7 was out and fully stable, we'd be at the mercy of the unity folks as to when they decide to upgrade that inside the unity engine itself; and then we're at their mercy as far as which GC they choose to use, etc.
Naturally, of course, they have the same goals as all of us do as far as performance and efficiency, though, so I'd imagine they'd go with the most efficient possible thing, so long as it is rock-solid. But for the time being, we're already on the latest version of Unity, and there's not much we can do beyond that when it comes to things of this nature.