Flatpak Steam users: The game has a hard-limit on the length of the installation directory. It's 59 characters, and a flatpak Steam install will probably exceed this. Create a new directory in ~/.var/app/com.valvesoftware.Steam named "Forza" and add it as a new library folder. Move Forza to that folder and you should be able to launch the game.

Other than that issue - seems to work flawlessly. I experienced some bugs in-game but I don't know if those were due to running under Proton or just scripting errors in the game.

Performance seems "normal" - I think the game is just unoptimized.

With the xpadneo driver, even trigger rumble works on an Xbox Series X controller connected over bluetooth. Doesn't work over USB for some reason, but I don't think that's a Proton issue.