TLDR; Since the One X/S now supports FreeSync and 120Hz give us the option to disable V-Sync.
I’ve played the beta everyday until the trial ended but due to the use of V-Sync I refused to buy the game, which is a shame because it had the best graphics on xbox until the recent BF1 update.
V-sync only forces your screen to show a full frame before showing the next one. So if you play on a TV which usually has a max of 30 fps you need your xbox to output more than that to make a difference.