Hi guys.
Yeah, I also did some testing last night and initially got the same result with game focus needing to be on the stereoscopic window and not the Mirror window.
I then plugged in a 2nd monitor, set the main Stereo window to focus on primary monitor, and Mirror on 2nd monitor, and it works perfectly!!!
No performance hit, runs as smooth as before.
And with the additional Mirror config options, you can get a nice high res, full screen for "passenger" viewing pleasure.
SOLVED!!!! Thanks to all for the help and suggestions (Well....not CodeMaster). We got there together in the end.
Hope this helps somebody else too.
(BTW, if you do struggle with frame stutter, this article solved it for me. It's focus is Non-VR, but worked in VR for me. Used to get a very slight glitch/stutter every 4 or 5 seconds)