HDMI 2.1 is required for 120Hz. If you aren’t using a 120Hz display, then HDMI 2.0 is fine (the standard) for 60Hz. Again, the 120Hz refresh rate is going to be really game-specific in terms of an FPS boost. Some games don’t utilize it except in very specific circumstances. I’ve tried to search for specific MSFS’ specifications for Xbox Series X and Series S, but I haven’t really found out if the sim is hard-capped FPS-wise at 30 FPS or 60 FPS on Series X. I cannot see how it could possibly run faster than 60 FPS.
I’d love some hard data from MS on this.