I can't find information on how to achieve monoscopic background with RT applied to a quad. I was able to do it without trouble some time ago then something went wrong. No matter what it try i get weird results. Is this a known issue with actual Unity (5.3.4p5)/OVR Utils versions, or does someone have some advices ?
Your approach sounds good for a space shooter. Performance-wise, your approach should be pretty similar to ours. You may just see a slight wobble in the background image until we share our sample. Your projection issues should be fixable today. Are you setting Camera.projectionMatrix equal on all your cameras?
I'm not sure regarding the projectionMatrix - I use the dual OVRCameraRig method now (near cam and far cam) and that works well enough, though the far camera rig only renders objects in my game's 'enormous object' layer. Yes I think it wobbled a bit - was a couple of months ago and I don't remember the details.
I might re-examine using a render texture when I've some time (or try Oculus' example if it becomes available first) as it may let me improve the graphical quality a little with the spare horsepower.
Near and far OVRCameraRigs should actually line up perfectly. It will only wobble if you use a "center eye" camera that renders only one image for the far frustum.