Sun Temple is probably the worst case scenario for reflections in VR, lots of contrast and shiny, flat surfaces. In a real game project I would strongly suggest disabling SSR and just designing your levels to look good with carefully placed reflection probes, because otherwise you’re shutting out a lot of people on weaker hardware. There’s also the distracting SSR artifacts in the periphery of your vision, which I personally find to be very annoying. According to the profile you posted your 980 is spending more than half the frame time on SSR, it’s just not worth it IMO, especially when you could be using that rendering time on higher screen percentage and better AA. But it’s up to you to make the decisions, you should pick a target GPU and make your game look as good as it can on that hardware while maintaining sub 11.1ms frame times.
To see a general overview of VR performance you can open the SteamVR settings->Performance->Display Frame Timing, which will show you how long it actually takes UE4 to render and what the rest of the system is doing each frame. UE4s GPU profiler tells you what each part of its rendering pipeline is costing you.
The checkboxes in the PP volume are for whether the volume should override that setting, if it’s unchecked like you said, then whatever the slider value is set to doesn’t matter because it isn’t being used.