Shield is really the way to go for me it sounds like. I'm not using a PC for any playback as I demand the ease of use that a built in dedicated player offers, so hopefully the Shield with Plex will work well for this. I need to know a bit more about how the remote works with the Shield to ensure that I can integrate it with the rest of my home system. We have standardized remotes throughout our home. But, that's more of a Shield discussion than a Plex discussion, and I have looked a bit into it and think I can get it all to work just fine.
Good to know about the support for 4K and HD audio. I do have my new JVC 590 hooked up, but no 4K capable receiver, so I will probably do a receiver upgrade at some point. I did pick up a Sony UHD BD player to do some testing of things with the projector... which looks awesome. But, for Plex, I'm still using my Roku Ultra which is tied into my system at 1080p. I have a few 4K 10Gb/s HD cards which could give me 4K/30 to my projector and my Samsung TV, but nothing else in my home supports that level of resolution, and I'm not sure if my scalers will take things down to 1080p. Guess I can find out, or perhaps push EDID on the fly to change the resolution of my Roku.