The nvidia-prime 0.5 release improves hardware detection, better deals with X.Org Server configuration files, and adds a prime-select switcher for switching between the integrated and discrete GPUs. This work also allows disabling the discrete GPU for power conservation. The nvidia-prime 0.5 package is used in conjunction with the NVIDIA R319 Linux graphics driver release or newer.
Details on nvidia-prime 0.5 that is available for Ubuntu 14.04 development work (and not Ubuntu 13.10) can be found via Launchpad.net. The update is currently in trusty-proposed.
An X.Org Server 1.15 development release has also been pulled into the Ubuntu 14.04 archive that includes upstream fixes for Optimus/PRIME handling and other fixes.
Stay tuned for my extensive NVIDIA Optimus laptop benchmarks under Linux using Ubuntu on the ASUS Zenbook Prime and it does include Windows 8.1 reference results.