Lets stop talking about the D800 as it its simply and solely a trophy to be compared, and start talking about it from a real world context. No one cares how it compares if you upscale a 5D III image to 36.3mp size. Neither does anyone who uses a 5D III or any other Canon camera really care how it compares if you downscale a D800 image to 22.3mp size. They care whats possible in the real world, with real-world software...tone curves and all.
real world it's simply not fair to compare cameras on a non-normalized basis to one another,it doesn't make any sense
And yet...no one actually lives in the limited reality wherein technical comparisons between hardware actually create photography. Sorry...people live in THE REAL WORLD, and in the REAL WORLD, people don't "objectively" utilize their cameras with unmodified, linear import to "see" all the dynamic range their camera has available. In the REAL WORLD, people apply base tone curves to their photos, to compress the considerable dynamic range...from either a Nikon or a Canon camera...which tends to be far greater than the dynamic range of either our computer screens or anything in print, into the much smaller dynamic range of those devices so our photos actually look good.
Tone curves are a real-world thing, they actually exist, and they are utilized by the very, very vast majority of photographers. The differences in tone curves and the distribution of levels in those curves between manufacturers is a meaningful topic.