Confirmation that the Rift will be running at 90Hz. Palmer Luckey on Oculus VR's Content-Focused Future - YouTube skip to 3:15
Ok then let us just hope that Epic can optimize this engine pretty good. At the moment you take a massive performance hit in VR mode. And fps are not that high to begin with in normal mode.
Otherwise I cannot see UE4 being a suitable engine for VR, if even a GTX 780 Ti fails to achieve enough fps in some demo projects. I think it is safe to say that the average consumer will not have a stronger graphic card by 2015/2016