But afaik you don't have to render the whole image twice but two halves once. Every eye only gets one half of the screen resolution, so together you render one full image. "Rendering in 3D" isn't a problem as every 3D game renders everything in 3D, for the Oculus you just need one more perspective. I think the main performance hit lies in the fact that this process isn't accelerated enough by the hardware(the driver you mentioned) and the additional image adjustments for the Rift.
I'm wondering if for the DK2 that it has to render the image twice and then rendering two more times for the colour off set in each lens, and that's where you get the performance hit.
It will get better in future with the hardware and software, may be having two separate screens each run from separate graphics cards.. Who knows ?