If you are talking about viewing 2D videos on the Pimax 8k, there is no "mode", you run an app that creates a virtual theater; Big Screen is a SteamVR app commonly used for that, or Virtual Desktop and just playing a movie in your player of choice. In either case, it would be the same as normal Steam VR apps - they should just (in theory) support the wider FOV out of the box.
Personally, I don't want extra FOV for a virtual theater screen, because that means there are parts of the screen you can't see clearly - it'd be like sitting in the front row at the theater. However, if the app actually placed you in a virtual theater with seats and stuff, that would be good for wide FOV to immerse you in the theater experience. Though I'm not sure if I would like that instead of just a screen hanging in blackness, for no distractions from the movie itself.