I watched the video, extremely good, extremely informative.
Yet even with more general knowledge about Windows' security model in my back pocket my opinion remains the same:
- Leave UAC on full
- Begin engineering work to move to a more fluid admin/user model (that doesn't require fast user switching etc)
- Warn the 3rd parties that if they aren't user-mode compliant by Windows 8 then they're in deep trouble
And in answer to the video, this doesn't add a new expensive security boundary. It relies on the admin/user model that is already in place but fractures user accounts into subsets with different permissions.