TwoLions

It isn't an excuse. I don't need one to give Canon a hard time.
 
Facts speak for themselves. 5D Mark III is $3000 and D5200 is $700. Look at the video. They're giving similar results.
 
I don't shoot with a flat profile on a DSLR it doesn't make sense. You trade important stuff like gradation, good skintones and colour for like ONE STOP extra dynamic range, and increase noise especially in a low light situation. It isn't worth it. Both grade similarly and have similar dynamic range - in the daylight shootout you will see that.
 
Clean HDMI on the 5D Mark III doesn't yet exist so until that day comes you're just speculating as to what it might be like.

I don't even understand why the question is so difficult to answer. If the 'capture format' is known that defines the outer limit of the color space, 4:4:4 being 100% uncompressed. Minute that you apply compression you can bet that color compression is going to be one of the losses, thus 4:2:2, 4:2:0 etc.
Unless your dealing with fully uncompressed color space there's not much point in talking about Chroma Key.
Let's focus on the overall look to the eye and the vectorscopes for grading in post.
Based on the side-by-side with the GH3 the D5200 footage looked visibly better in most shots with less obvious macroblocking in the deepest values.
We've been through this exact discussion before, when the Nikon D800 was first released.
Bottom line is we're looking for RAW out of HDMI.

@QuickHitRecord
Pretty sure what you're seeing isn't the result of the color space, it's the result of the massive overall bitrate increase going from H264 to ProRes.  Detail and clarity come from bitrate increase, color data change is more hard to discern.