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).
Yeah, love it. I mostly use it to manage games that misbehave when losing focus or the games that try to prevent me form alt-tabbing. Just pop them into Gamescope, and alt tab to my heart’s desire.