You'd still use a 5k display in a non-native 4k resolution (surely the xbox doesn't support higher resolutions?), which might be ok for video and most games, but it's not a good solution. I'd just get a half decent, affrodable 27" / 4k display for gaming and keep the Studio display for actual work.

Plus, you can use the additional 4k display as an extended macOS display when not gaming. I mean there's never enough screen real estate.