I have the Vive Pro too (and love it) and set the Steam SS to manual overide. I set it in video to 100%, then adjust it per application in Steam VR, a new setting which is fantastic!
Some games, Project Cars 2 for example, I lower the SS in Steam VR application to 80%.
However, 80% SS on the Pro is equal to the original Vive with a decent amount of SS added. Don't expect all games to run smooth at 100% and 100%, it is asking a lot!!
Skyrim VR on the otherhand I have at 100% and 120% I believe, and despite all the mods I installed, it runs so smooth