Last week I received my valve index and I have been doing several tests with DCS and my rig (9700K, 2080Ti and 32GB). My configuration is SteamVR SS of 200% with MSAA set to x2, and the refresh rate to 120Hz.
If I disable motion smoothing, I get a mean frame rate of around 65 FPS flying over Dubai, but when I fly low, I see some kind of ghosting effect in the buildings and vehicles.
On the other hand, if I enable motion smoothing feature, this ghosting effect nearly dissapears, but I start seen some kind of jumps inside the cockpit and the builds, which are very erratic. The performance for this case is nearly 40 FPS, but running quite smooth.
Are you experiencing this effects? Maybe the problem could be the reprojection? In my case is enabled (I don't know how to disable it).