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 launch it from a TTY to be able to use HDR on my monitor. As far as I know it’s the only way to get games to use HDR on Linux. However using Gamescope in embedded mode clashes with Wayland, so I have to have my regular KDE desktop on X11 to do this.