I ran into an issue where I had high cpu use sending a 1080p/h265 file to my new ChromeCast Ultra from Videocast. I took a random guess that it was due to transcoding.
From there I spent time researching the underbelly of ChromeCast detection. Yeah. It's non-awesome.
I needed to be able to test, so I did some testing with UniversalMediaServer.
http://www.universalmediaserver.com/forum/viewtopic.php?f=5&t=9130
TL;DR: UserAgent can only be differentiat by cpu arch, but the U will handle my 4k/hevc files fine with 0 transcoding.
So I guess my support request is to detect the CastU and passthrough HEVC to it without transcoding.