No need for that overhead, simply disallow untrustred clients, trusted clients that explicitly request to drop down to untrusted (e.g. browsers could do that) and clients the user explicitly marks as untrusted to access other clients' data. Xorg already has most of this functionality (AFAIK all remote/socket connections can be made untrusted) and last time i heard about it, it is something some X developers are going to look into improving.

This way you don't have any overhead, you preserve backwards compatibility with the protocol, existing programs that need the functionality of accessing each others' data (window managers, panels, tools like xdotool, screenshotting tools, etc) can keep working like they always were and you still get to implement sandboxing (with or without snaps/flatpack/etc) without breaking the Linux desktop.