What Compression does HERO3+ Video Playback and Live Preview use when Sent Over HDMI?

For live feed, HERO3 cameras output uncompressed video over HDMI output.
 
For video playback, video is stored compressed on camera but upon playback video is uncompressed and output over HDMI.

 

How Much Latency is there in HDMI Live Preview from the HERO3+ Camera?

You will generally see about a 2-3 second delay when using HDMI live preview.

 

What's the color space of the HDMI output (i.e. RGB)?

The color space of the HDMI output is RGB in Rec. 709.