Yes the Cloudstor can definitely stream HD MKVs. I have streamed 10+ GB videos, including MKVs, on my local network. I don't know if my internet connection would be able to provide the necessary upload speeds to stream these over WAN though.
Also, unfortunately, I have only tried to stream videos from it over WAN on mobile devices, namely my HTC Evo smartphone and my Motorola Xoom tablet. Both of those are limited by their software and Wifi radio speeds. So, while standard definition videos stream onto those devices wonderfully, the high quality stuff is another story. For one thing, it usually requires transcoding (and I NEVER EVER transcode using the built-in pogoplug transcoding - but that's a totally different story.) My Xoom can play beautifully high-quality video, but it usually needs to be encoded with specific settings -- and even then HD content usually only works smoothly when playing locally from the device's storage.
But I am quite confident that this is just because they are mobile devices, and that if I were to use a laptop, for instance, it would work great. The only reason they wouldn't play well is if I didn't have a strong enough internet connection.
