2016 13" MBP + RX480 + Sonnet III-D + OS X 10.12.2
@itsage Thank you for the update! It's a shame that you are also having issues with 4k playback on both your machines! It's help with my suspicion that Nvidia cards aren't playing too well with either the hardware or Sierra itself!
@goalque Sorry that I forgot to mention that I have done both of those as well with the Titan, and I still had no luck whatsoever.
@Anchoricex I knew people were having problems and I'm actually expecting it to crash at some point because I haven't used it a lot just yet. Conversely, with my 2013 15" + Titan setup I was getting crashes getting Premiere Pro to use the GPU.
VLC for Mac supports only H.264 video decoding acceleration:
Some samples at 4ksamples.com use H.265 codec. H.264 worked pretty well.