OK I appear to have solved the issue.
after several un-necessary reinstalls and a lot of searching, I appear to have stumbled across the fault by accident.
so it may have been this or a combination of things I had setup that it didnt like.
My IL2 game is on steam. Just for the record.
I reset and re-installed Oculus Tray tool, and made sure ASW was 'off', (supersampling on OTT is PC dependent, but I have that set to 1.5 in any case) still wasnt working, adjusting IL2 settings provided a few extra frames on its own by configuring settings, but not enough for smooth play.
Seeing as there are many ways to boot a VR/VR supported game, and Oculus software relies on Steam Vr also. I started experimenting with different boot up methods. instead of booting through Oculus and allowing that to open steam, I loaded steam and Steam VR, and booted it IL2 through Steam VR. instead of relying on Oculus to start the process.
Seems Oculus software failed to load Steam VR correctly somehow (Just for IL2, all my other games and apps work fine booting through Oculus), my PC appears to have been treating IL2 as a background task. even though in task manager, it was on the primary list.
I have reset all my Steam VR settings to default, and am using OTT now to configure the sampling and graphics., so VR resolution in Steam is just set to 100% even though my PC can totally handle more.
So the process I followed was:
Open OTT (Tray Tool)
Open Oculus
Boot up Steam VR in Oculus
Load IL2.
Its a faff, but that process appears to be working.
happy 80FPS all day today...
I have also managed to max out the settings in IL2 as well, and it still runs 80fps, so its 100% a software issue and not hardware. However, and I never fully understood this, but in the IL2 settings I am now using MSAA, instead of FSAA.