So with Unity 5.2 multithreaded rendering works again! Plus, Google added time warp to latest SDK (not for Unity yet). I am just hoping UE4 will not drag behind Unity/Google on this project.
Btw, there is also this sweet app: https://play.google.com/store/apps/details?id=org.hitlabnz.sensor_fusion_demo
Maybe it can be integrated into UE4 to provide spot on tracking for Cardboard. It’s source code license allows integrating it into proprietary engines.