Sometimes. It contains fixes for specific games and patches from wine-staging (the newest, testing branch) that are not included in Valve's Proton releases, many of which have a positive impact on performance. The project's overview is the best summary.
Or for in game cut scene/Media sometimes it is a must: Additional media foundation patches for better video playback support.
It fixes issues with games. If the game has no issues with regular Proton then the answer is generally "no".
In fact, with regular Proton you're much more likely to have pre-compiled shaders available for your system through Steam, which should improve performance by reducing stuttering.
Depends on the game. For example I can’t get Persona 5 Strikers to run in Proton. No videos load or cutscenes. I’d I run it with Proton-GE (9.x), it works flawlessly.
