I installed the latest Unity 5.4 Beta last night which is now RC1, everything seems to be working OK. I need to re bake all the scenes and then I should be able to make a new build that has native OpenVR included.

I had a look at integrating the Lab Renderer but I couldn't get it to work properly and it requires OpenVR exclusively which isn't something I am willing to support at the expense of the Rifts ATW.

There should be some perf improvements from 5.4 Single Pass Stereo but on a min spec VR machine you're likely to average around 70+ FPS. Without the Rifts ATW I'm not yet sure how juddery this will feel.