I hope I understand your questions, I had a bug that locked the Vive at 60 fps and I found that the experience was very unsettling, the 60 fps was based on the refresh rate of the monitor not the HMD so it should be somehow possible to lock it but I think 90 fps with more optimized content would be better.
In reference to the DK2 I couldn’t agree more that the 75 fps seems pretty good however I’m gong to assume that timewarp and direct mode probably play a big part in creating a feeling of higher fidelity fps, if timewarp creates a dummy frame between each frame then we could be talking about a 150 fps feeling.
I’m not sure if the Vive drivers/sdk provides a frame projection solution yet and it looks to me that it is still using an extended monitor styled setup so that could explain many of the differences you were talking about.
Out of all the time I’ve spent developing for VR hitting the recommended framerate always seems important as many other factors rely on that refresh rate, like timewarp etc. Optimization, low draw calls, optimized texture sheets are just a few ways to get that desired framerate.
If you haven’t already there are presentations where Epic provide a number of their optimizations to get their VR scenes running smoothly, I use these and haven’t had too much trouble hitting the desired framerates:
Hope that helps