On my system the rendering times are overall quite sub par compared to VP11. This is due to poor use of the GPU.
Certain settings for memory in options (10 gig of 24 gig) will make rendering to Mpeg-2 with 2 pass better in VP12. The second pass speeds up dramatically as if it is no longer using the GPU or whatever at that point.
I cannot remember the driver version number I am currently using I think it is 290.10 but VP12 works best with it and does not crash but is still overall sub par compared to VP11.
With later Nvidia drivers rendering will not even complete and VP12 will crash. Also VP12 will crash on exit even if nothing is done with it.
If SCS ever releases another build for VP12 then maybe there will be a difference? MAYBE!!!