Plex insists on transcoding videos that are H.264 High 10 on the Chromecast with Google TV. It does support the H.264 High 10 format per specs:
Chromecast with Google TV
- H.264 High Profile up to level 5.1 (4Kx2K/30fps)
- HEVC / H.265 Main and Main10 Profiles up to level 5.1 (4Kx2K@60fps)
- VP9 Profile-2 up to 4Kx2K@60fps
Here is from the dashboard:
VIDEO 1080p (H.264 High 10)
AUDIO English (FLAC Stereo)
I really want to get a 4K TV to use with Plex but without proper H.264 support the file sizes will be insanely huge.