Google have updates their documentation regarding supported codecs and containers on Chromecast, for both 1st/2nd gen and Ultra.
ATM, Plex transcodes codecs we now know are able to play without it. It degrades quality, and uses unnecessary processing power. It's about time the Chromecast.xml gets updated to reflect that.
And maybe you should differentiate normal and ultra too? I know stuff like this should be posted in the feedback section, but nothing ever happens to the posts there.
Here is the info regarding supported codecs and containers:
Google finally say they support Opus audio codec on all chromecasts, and Ultra supports HEVC and VP9.
Normal Chromecast also support direct playback of VP8. Chromecast.xml does not reflect that.
I only own the regular Chromecast, but I'm going to try to get all supported formats to play without transcoding when using Plex. If anyone got this working already, please share the xml. :)