I know this isn't much help, but I also have a 1080 Ti and I am getting pretty much constant 165fps at max settings at 2560x1440. I don't think Steam is the issue.
this is weird, I have no idea what the issue is then. I'm seeing in these forums I'm not the only one who is experiencing this. Do you happen to run Intel or AMD? If Intel are you dedicating your iGPU to run other apps independently of your games so yours games aren't affected by other apps encoding while playing?
I have a 6700K and I don't dedicate my iGPU to do anything at all. Nothing fancy or anything like that. Just stock settings and running the game normally.