As soon as ...
- ... libav / avconv is able to produce valid WebM files with VP9 video stream and opus audio stream (c.f. bug 53863 comment 8 )
- ... and the support for WebM VP9/opus is "high enough" (c.f. http://caniuse.com/#search=webm shows currently only 60 % of "all WebM-devices" have full WebM support)
it should be considered to also produce WebM VP9/opus transcode-derivatives through TMH.