My understanding is, that although the game renders two views for VR purposes, it is still rendering one frame for both of them at 1440p.
If that is the case, I will check my frame rate and report back. I have a 1440p monitor and 970 sli.
Edit
Fraps is reporting 190 to 210 fps at most places...
Does anyone know specific instances where the game gets very demanding?
It only renders one frame for both views so you've got:
a) double the POV (geometry)
b) substantially larger (maybe 2x) frustrum (more geometry)
c) a higher resolution pre-warp buffer in order to compensate for the barrel distortion that stretches the center portion of the image (~2x pre-warp samples to get 1x post-warp samples in the center of the POV)
d) that buffer is actually an over-sized buffer in order to support time-warp (so +% FOV and samples)
e) no frame queuing/pre-processing
f) many people actually add yet more super-sampling on top of all this due to the difficulty in reading the text on HUDs, aliasing, etc
g) vsync
With the Rift the consistently low spots seem to be inside busy stations. I think(?) you can enable the Rift mode in ED without a Rift plugged in or any oculus software/SDK installed. So you could set in nv control panel your max pre-rendered frames to 1, force vsync off (i think ED forces this on in the Rift mode), and see what sort of performance deltas you're getting between 1440p monitor and a "1440p" rift-mode. For myself with a single 970, I'm able to play on my 1440p monitor on pretty much highest settings and see solid 85-120fps in stations, but in the Rift at "1080p" I need to crank everything down, and even then I still get lots of dips below 75fps inside stations.