Yes, SteamVR shims itself between the game and the Oculus Runtime in some manner, trying to use as much of the functionality of the latter as possible, over its own (including reprojection).
None the less, using SteamVR with an Oculus Rift is a bit of a nightmare, as the two interfaces fight it out over resources, settings, overlays, inputs (the conflict with menu buttons, alone... Argh!), etc.
It's a beginning, thanks!
Would really like to know how the interactions go, and why presented numbers ostensibly differ from actually used ones (SteamVR, after all, tells the game what size bitmaps to render, when SteamVR is the API the game is written against) (...or maybe they don't differ, and things are really rendered at an insanely high resolution, resulting in the expected performance hit, and then it's just a matter of the difference being unnoticeable due to diminishing returns), and just when and where, and how, under what conditions, overlays from the SteamVR compositor (...which has its own, separate supersampling value), gets drawn in (chaperone, control/store panels, controller models, etc).
"3" on the Piplay Quality slider means make the render target three times normal width and three times the height, does it, or it it something else?