“HMD Quality” in Elite Dangerous is an additional multiplier, on top of Recommended Render Taget Size, and unlike “new style” SteamVR supersampling settings it factors the axes, so x2.0 is X*2 and Y*2, and consequently four times the number of pixels to render (…or 400% in newspeak SteamVR – doubleplus unconfusing :P).
“Supersampling” in Elite Dangerous also multiples the dimensions in the exact same way, and renders that larger bitmap, but then the game on its own downsamples the result to what was originally asked for, prior to that increase in size, before handing the images over to the VR runtime (e.g. OpenVR). This option is also present for rendering to monitor, and that’s really what’s it’s for. It can be useful in VR if one want to push past a combined HMD Q and SteamVR SS of 500%, because at that point, texture filtering will begin to skip samples from the game’s rendered images, rather than including them all in its calculations, so in that case, it can be useful to have the “top load” of detail “prebaked” into the input image – too bad one can not get reasonable frame rates at that point.
(EDIT: …should probably mention that to my eyes, just as logic would suggest, there is no differerence between SteamVR at 100% and HMDQ at 2.0, and SteamVR at 400% and HMDQ at 1.0 – they produce the exact same thing.
What does make a tremendous difference to me, is to opt to use SteamVR’s older filtering method (“allowSupersampleFiltering” : false), which results in a significantly sharper output, at the cost of not smoothing out aliasing as much as the newer one (EDIT2: Some prefer the opposite tradeoff, and will take blurry imagery, if that means less aliasing -. these people are wrong. ))