This has nothing to do with SteamOS, X11 or Wayland. Game performance on Linux has everything to do with the quality of the port and/or game engine and hardware drivers. For example, Rocket League performance on Linux is on par with Windows while Tomb Raider has 20-30% less. In any case, there is no perceivable difference if you have a modern $200 GPU.

BTW, X11 with DRI3 has the same performance as Wayland.