Haven't had any issues with the NIC at all. I haven't played any MKV's but can run a test tonight. Will use the birds test from http://www.auby.no/files/video_tests/ unless you have something else you'd like me to use. I'll try to get the temps while it's streaming as well.
I'm using the latest release candidate (5.0-rc8a) with only Simple Features and unMenu for addons.
I ran the birds test video and I'd have to say it went really smooth. I've uploaded a video of it here. This is played in XBMC on a full Windows 7 install over samba. Sorry for the poor quality but all I have is my phone. FWIW, in the video I uploaded it seems to start out a little slow, that must be from my phone's camera because watching it live it looked perfect. I also ran the video 4 or 5 times on my main computer, no stutters or issues at all. I checked the temp immediately after and it was at 63.1 C. For reference, it's been probably 1/2 hour since I ran anything off of that server and the temp is sitting at 62.0 C. Overall I'm extremetly happy with this little guy.
Anything else you'd like me to check out, let me know.