At present, only the Nouveau and AMD Radeon and Intel open-source graphics drivers are compatible with Mir when running a patched Mesa that supports the Mir back-end. Even then though, the Mir multi-monitor support is acting up at the moment when using non-Intel hardware.
Anyhow, during the Mir/XMir Quality and Performance Benchmarking for Saucy vUDS session, it was brought up about testing Valve's games on Mir and then more broadly the binary AMD/NVIDIA drivers.
In terms of Valve game testing by Canonical, they aren't too concerned about testing Valve's titles at the moment since the NVIDIA and AMD binary drivers don't yet support Mir. Those drivers for now will be falling back to using a pure X.Org Server configuration. The engineers believe those wanting to play Valve's latest AAA titles on Steam will be using the proprietary drivers. (Plus there's other issues in benchmarking Valve/Steam games.)
Anyhow, the matter of binary drivers supporting Mir was then brought up. Long story short, Canonical is still engaged with the graphics hardware vendors about supporting Mir, but they won't reveal their "partners" road-maps/schedules and information. Given the Ubuntu 13.10 feature freeze is this week, it's unlikely to see any AMD or NVIDIA binary drivers in Ubuntu 13.10 that properly support EGL/Mir. However, given that they're still engaged and that the (non-X) Mir will play a larger role in Ubuntu 14.04 and it's also a Long-Term Support (LTS) release, April may be a better time-frame for expecting support from the binary blobs