It shouldn't be that bad. Compositors already try to get out of the way of fullscreen windows. We already have VRR working great, been using it for months and it's absolutely flawless. So if the game just needs to push out more (generated) frames, it can probably work fairly decently as-is.

Vsync and perfect frames was one of the original and primary goals of Wayland. I haven't had weird stutters and frame pacing/timing issues in a really long time, at least on AMD.

If it needs compositor level changes, I think we're probably not too far. And the feature needs to be out so compositors can implement necessary workarounds anyway.

In all cases it's probably better than nothing.