Hi there
VR is hugely demanding, having played Elite Dangerous on the HTC setup I was totally blown away, in that game VR is a game changer, it is completely immense. When I asked HTC what is the minimum specification they were like the MINIMUM is 970, but right now you are playing on Titan X's simply because in Elite a 970 means detail levels have to be reduced to keep VR smooth.
In short you need a 970 minimum, if you want maximum eye candy you need 980Ti/Titan X.
VR demands are very high and to properly enjoy it in games like Elite and other titles coming for what I consider a good gaming experience, so all detail settings on high/maximum then a 980Ti, Fury X, Fury or Nano is best.
You will get away with a 970 or 390, but in some games you need to turn details down, on slower GPU's and older GPU's well I'd not be paying £600-£800 for a VR headset to then only enjoy it at poor FPS and low detail levels.
So can't wait for Valve to release Half Life 3 VR Edition, oh my, please make it so!