According to YouTuber Dave2D who managed to get their hands on the Lenovo Legion Go S, the SteamOS version destroys Windows in performance and battery life.
When it comes to battery life, SteamOS was yet again a very clear winner in this with even demanding titles like Cyberpunk 2077 having 1 hour 31 minutes on Windows and 2 hours 6 minutes on SteamOS. In less demanding titles like Dead Cells the difference is absolutely insane jumping from 2 hours 47 minutes on Windows to 7 hours 8 minutes on SteamOS.