Have you tried it? I just installed gamescope and launched some games with it via steam launch options. So far I feel like the frame times are better. I use it on top of KDE and with an AMD card. Freesync in Gamescope works too. (Have it allways on in KDE if that matters).
I do… for playing games in 4:3 stretched. Don’t use any of the other fancy tools. Recently it has been breaking and is crashing games on launch from using -f and mangohud. Not sure what happened there.