I ran through several KA-50 Normandy Free-Flight missions starting with the VR Presets as baseline settings and a Pixel Density of 1.5.
VR Preset: 45 FPS (Deferred Shading On, HDR off, Medium Shadows, Flat Canopy Shadows, etc.)
Change: Anisotropic Filtering 16, Global Illumination On, Dirt+Smoke Effects: 45 FPS
MSAA x4 and Pixel Density of 1.5: 28 FPS
MSAA x4 and Pixel Density of 1.0: 45 FPS
VR Preset + ASW Manually Disabled: 45 FPS
I'm pretty sure the 45 FPS ceiling is due to ASW being enable. Even when I forced it off in the Oculus Debug Tool, it started off disabled (in the DCS menus) and then (magically) became enabled after loading the mission.
The only time I struggled constantly for frames was when I was using MSAA and a pixel density > 1.0. On a side note, flame/smoke effects also caused temporary dips in FPS (~35 FPS) from crashing (haven't been in the KA-50 in a while) or the couple things I managed to blow up while running around Normandy like a mad-man.
I know from some previous discussions you're making use of the Oculus Tray Tool. It might be worth disabling it temporarily, verifying that Anti-Aliasing is disabled and try again.