Perhaps the most interesting use of the Series X's capabilities in backward compatibility mode is found in Rainbow Six Siege allows the gamer to turn the vsync completely off, and furthermore, it also displays its own custom frame rate counter, in game! Check this Digital Foundry video out where they're playing the game between 120 and 134 FPS!?
https://youtu.be/LsQldhmN5XU?t=601
Oh how I'd love to have the option to set vsync at 30 Hz/60 Hz/OFF, with a frame rate counter for every sports game after seeing this!