I'm actually thinking about this now, but only for 4k content as I'm perfectly fine with x264 for 1080p and lower material.
The reason I'm thinking about it is mainly because of PGS subtitles of all things. In my method for Blu-rays, I OCR the PGS subtitles and create a text based .srt file. That way, optional subtitles can be used on almost any player without Plex having to re-transcode the video. The player can also re-size and sometimes color them.
I only have a few 4k rips, and was planning to just keep them as full sized remuxes (they are now), and do the same for their optional subtitles (OCR them to soft .srt).
The issue is that many movies have a forced subtitle track with only a handful of subtitles to translate a small part of the film. These really should be burned in, but they aren't so the disc can accommodate multiple languages. The problem is unless you are using something like the Plex Media Player on a computer to view the movie, Plex will need to transcode the entire film just for those 3 alien language subtitles.
For my 1080p movies, since I'm transcoding them at a high quality before putting them on Plex, I just burn this forced PGS track. You could OCR this track, and then tell Plex the soft .srt file is forced. I do this with foreign language movies. The problem when a movie places the subtitles over a certain part of the screen (The Shape of Water is a perfect example of this), or the subtitle has a nice font to go with the film (The Godfather). Soft subtitles can generally only be placed at the top or bottom of the screen and the font is chosen by the player.
So all that being said, I think I'm going to end up transcoding even my 4k rips, just at a really high quality. I'll end up saving a little space and have a file that's more ready to view on its own with the forced subtitle issue out of the way.
My build from earlier in this thread is still working great, but thinking about a bigger build in the near future. Thinking Intel's Ice Lake Comet Lake may be out by then and already have an idea of what case I want to use. Using my preferred encoding settings, I can only get about 1-2 fps on a 4k encode with my i7-6700 right now.