I have backed the Pimax 8k recently. They are having problems hitting 90 Hz and they don't know why. All of the parts they use are 90Hz certified but there seem to be some lingering compatibility issues... They had currently verified 82Hz on the Mk3 concept last week and say that 85 is currently under test... They also say that they want to release 5 concepts before the final build.
The Mk.2 which Linus tested was rumoured to be running at about 75 which even with just 100° FOV is the absolute minimum. And you know it's going to be an issue when a battle hardened VR soldier like Linus lasts a mere 5 minutes.
Knowing that wider FOVs exacerbate dizziness issues, 90 is really a bare necessity rather than icing on the cake...
I'm sure that future HMDs by other manufacturers are going to be pushing 120 or even 144Hz in the quest for a silky smooth experience.
Still as a cockpit experience, ED should be fine @85Hz if the positional low pass filtering is not too extreme. SRV might be a stretch, though. That can be a barf fest at the best of times.
edit-
I, personally would be far happier for them to miss the deadline even by an entire quarter if it means they'll deliver a quality product... But then I backed SC, so I know what the word SOON™ means.