Hello,
I just tested both the Rift S and the Valve Index in VR and HDRP.
Using version unity 2019.2.0b10 and HDRP ver. 6.9
HDRP looks really nice in VR the only problem is that the framerate in OpenVR is dropping to half after some time, while the Oculus SDK runs fine constantly at 80 - 90 fps.
When the Oculus is set to Open VR SDK it shows the same behavior framerate drops to half after a while. Even in an empty scene, no volumetics enabled.