Replying to myself here, because I should probably include that even so, the VorpX flat-but-otherwise-"full VR" option is in my opinion way, way superior to Frontier's virtual screen: The headtracking is half the VR experience right there, and filling the full view of the HMD makes all the difference on any of the worlds in the Adityan system -- 'don't mind a pinch of image sharpening either. :7
I also always, personally, have the experience that more FOV is much more comfortable and less nauseating, than a more restricted one, contrary to the popular anti-nausea measure of pulling in the FOV during yawing, seen in many games.
The problems that crop up, is that there is no GUI scaling, so to be able to read menus, one have to hop into edge-peek mode now and again; And that the game seems to like to cap the resolution to one's monitor resolution, even in windowed mode. I got around that somewhat, by activating some DSR virtual resolutions in NVidia settings - it seems to me that VorpX captures the frame buffer before the downsampling stage. DSR x4.0 still looks a little too low res to my eye, but my 1080Ti is already stuttering badly at that point anyway.
I'm setting the FOV in Options/Graphics/Settings.xml to 110° for my Index, which seems a fairly good fit.