Like I mentioned: I have heard talk about Reverb users not getting the sorts of results they should rightly be able to expect, recently, and have seen through-the-lens macro screenshots, where the person taking them claimed to have used settings that should provide excellent imagery (...and does on their pictures from other headsets), but where you can clearly see the same pixel repeating in 2x2 pixel blocks, as if upscaled from half resolution, using a low-fi resampling method.
That high base resolution of the Reverb should make better use even of lower resolution input images, but apparently people have just not been seeing it, even when greatly supersampling.
So I could imagine maybe there has in the past been something wrong somewhere along the way, from the Reverb WMR drivers, to the WMR framework, through the WMR SteamVR driver, to SteamVR, and the way each of those speak with one another - perhaps throwing away stuff, or resampling it, at some point, so that you have had worse performance than you should
before, but are up to nominal
now, (EDIT: ...after an update to one of those components). :7
I'd like to know actual technical reasons, though. When people say they not only get something for nothing, but absolutely staggering results, outshining anything anybody else has experienced by orders of magnitude, I tend to assume there is a placebo effect in action...
(EDIT2: I just noticed in your first post, you said not only to supersample at 400% SteamVR general, but do it the same with the application-specific setting, which is on top of the main one. That would be 16 times more work than no supersampling at all, and would definitely be capped - especially with the Reverb, that should start out with a really high base render target (something like native resolution times 1.4, if the default is chosen by similar logic to the Vive).

)