It's the old “compatible with parallel projections” system, which we already knew reduced performance; but as much as 35%? Perhaps Pimax have optimized.
What they're not talking about is the "Hidden Area Mask" checkbox, which I think reduced performance more and was only necessary to fix artifacts in a few games.
By the way ¿are you going to try two 2080 ti in SLI? You've already seen in our game datasheet the few games that take advantage of it, haven't you?
Games with SLI:
EVE: Valkyrie – Warzone
NVIDIA VR Funhouse (needs another nVidia 980 ti or higher card por PhysX)
Prepar3D: Lockheed Martin (only without Flyinside)
Serious Sam 3 VR: BFE
Serious Sam VR: The First Encounter
Serious Sam VR: The Last Hope
Serious Sam VR: The Second Encounter
The Lab (only Robot Repair)
The Talos Principle VR
Trials on Tatooine (Star Wars)
X-Plane 11 VR (only without Flyinside) In theory it is prepared for SLI, but in reality it makes it perform worse, so I remove it from the list.