@Clmcm400 's code worked for me to get HEVC/H.265 directplay working on Chromecast Ultra too! Thanks a lot @Clmcm400!
I had to edit it slightly, maybe due to an update. Here is my full Chromecast.xml file, direct playing HEVC without transcoding as of Mar 1st, 2017:
<?xml version="1.0" encoding="utf-8"?>
<Client name="Chromecast">
<!-- Edited with Clmcm400's code from https://forums.plex.tv/discussion/244663/chromecast-ultra-profile-doesnt-support-hevc/p2 -->
<TranscodeTargets>
<VideoProfile protocol="http" container="matroska" codec="h264,h265,hevc" audioCodec="eac3,ac3,aac,mp3" context="streaming" />
<MusicProfile container="matroska" codec="mp3" />
<PhotoProfile container="jpeg" />
<SubtitleProfile container="ass" codec="ass" />
</TranscodeTargets>
<DirectPlayProfiles>
<VideoProfile container="mp4" codec="Hevc,mpeg1video,mpeg2video,h264,mpeg4" audioCodec="eac3,ac3,aac,mp3,mp2"/>
<VideoProfile container="mkv" codec="vp9,hevc,h265,mpeg1video,mpeg2video,h264,mpeg4" audioCodec="eac3,ac3,aac,mp3,mp2,pcm,flac,alac" subtitleFormat="srt,ass"/>
</DirectPlayProfiles>
<CodecProfiles>
<VideoCodec name="h265,hevc,vp9">
<Limitations>
<UpperBound name="video.width" value="3840"/>
<UpperBound name="video.height" value="2176"/>
<UpperBound name="video.bitrate" value="75000"/>
</Limitations>
</VideoCodec>
<VideoAudioCodec name="*">
<Limitations>
<UpperBound name="audio.channels" value="6" />
</Limitations>
</VideoAudioCodec>
</CodecProfiles>
</Client>