I was using the GPU usage shown in MSI Afterburner. By 30% headroom I meant that my GPU was mostly sitting at 70% utilisation although this wasn’t in a ‘heavy’ area. The Oculus performance overlay usually says something crazy like -150% headroom with these settings, I believe it’s because it is set to measure how far below/above 90fps you are rather than how much of your GPU you are actually using.
I believe the jittery cockpit is a bug with ASW and how it is interacting with OpenXR, I can get rid of the jitter by locking the game to 30fps and turning off ASW but it isn’t very playable like this (for obvious reasons) but frame-timing is a perfectly flat 33.2ms constantly. If I keep the same settings and just switch ASW back on the frame timings start jumping all over the place and the jitter re-appears. There is a thread about it over here, unfortunately it seems like something Oculus or Asobo must fix.