I got different results using another cable and plugging into my receiver. I was able to select 12 bit (but not 10) in multiple color modes. Pretty sure it's the cable, but my Shield let's me choose all the 8, 10, and 12 bit color modes without issue. Pretty sure it's the same HDMI cable too.
Makes me remember why I quit messing with HTPCs...
I will say, video quality is fantastic with madVR. Always has been.