Try setting the Index refresh to 90Hz, then in SteamVR lock the framerate to 22.5fps with additional prediction of 22.2ms, with Motion Smoothing enabled. This is giving me a really smooth experience, without excessive Motion Smoothing artifacts. Even with such a low framerate cap, with the smoothing I very rarely notice anything inside or outside that is enough to break the illusion of fluid flight, but of course it is ultimately a subjective experience.
If you haven’t done so already, I would check out @CptLucky8 's My 2070 SUPER VR settings and suggestions (Index - SteamVR) thread. Following most of his suggestions has really improve my overall VR experience with the Index.