This does not mean that there won't be games perfectly playable using for example ss 200% on the Index, even using my oc'ed GTX 1080 - and there are many simple games working fine. So if you just barely get 90 fps using 1080 Ti in a Rift-S game, you may - more or less - get the Cosmos and a 2080 Ti and just barely get 90 fps. These numbers may not translate directly to fps, but it's probably close. Ss 2.0 corresponds to SteamVR 200% and is equal to 4 times the res. Again 2080 Ti will only give you 30% more fps compared to 1080 Ti - it may easily be eaten up by ss. Yes, 2080 Ti is just 30% faster than 1080 Ti, and maybe fun with some res and numbers:
Bear in mind a 2080ti is a 30% performance improvement over a 1080ti.