>>341146293
If you already own the RiftS and 6900Xt you can try DCS for free, just fly around in one of the trial aircraft training missions or look at the fps on the ground.
I think RiftS would be adequate for IL2, try it with your current setup before upgrading.

Run it on high preset, check some guides online on what settings to knock down.
Off the top of my head don't go hard on in game AA at most MSAAx2, if you are taking a huge hit use FXAA instead and raise SteamVR supersampling a bit if you want a bit more clarity for text.
Don't go full ham on the supersampling as the game has shimmer for spotting distant targets that gets reduced if you supersample since supersampling is applied after the shimmer effect not before it.
Clouds high
get rid of grass, it's ass
tweak draw distance to your liking I think you should be fine cranking almost everything to high on a 6900Xt with a RiftS similar to vive Pro.
Use FPS VR or equivalent to fine tune your settings inside VR and see what the performance impact is.