Here's something I'd generally be interested in -- but, please, only reply if you have real-world experience, and not if you're only projecting your expectations from interpretations of an unfocused high-ISO macro shot of the inside of soWmebody's lenscap you saw posted on the 'Net.
In what real-world situation does the 5DIII have insufficient image quality for the job but the D800 does have sufficient IQ? That is, when would you put down the 5DIII and pick up the D800 instead of a medium format kit?
I personally can't think of any.
Landscapes...in a heartbeat! The average sunset landscape can have a DR well above 12 stops, even above 14 stops. The more DR the better for landscape shots. Not that you will necessarily use all of it in the final product, but that you can always use the leeway when it comes to landscape shots. Being able to do even a touch of shadow recovery where necessary without encountering the color-blotched noise-infested pattern-ridden shadows that Canon is well-known for would be a godsend. Not to mention the improved sharpness on the D800e.
I'm not suggesting that improvements are unwelcome or claiming that the D800 doesn't hold a slight edge in certain areas.
But are you really shooting landscapes in situations where you'd be able to tell the difference in the final print between the 5DIII and the D800, short of examining them with a loupe?
And if your landscapes won't sell with the 5DIII, would you really be able to sell them with the D800 instead?
That's my point. If the 5DIII isn't good enough, the marginal improvement the D800 might be able to offer really isn't going to do the trick, either, even if it's better. If you can't cut the branch with a hunting knife, a Bowie knife isn't going to cut it either; you need a saw or a hatchet.
shooting distant birds (high density sensor puts more pixels per duck)
Does the extra density, combined with noise characteristics and AF performance / frames per second of the D800 really result in a better print once both have been cropped, even if you've got more PPD from the D800?
Distance-limited telephoto (or macro) shooting is about the only scenario I can think of where the difference might be discernible, but I'm still skeptical about whether or not there'll be a real-world difference in prints based on sensor characteristics alone. I'm even more skeptical that the non-sensor bits of the D800 will let it make as good a use of its sensor as the 5DIII will of its.
for making 13x19" or larger prints
Eh, 13" x 19" is puny. If you're buying these cameras to print at 13" x 19", you've wasted your money.
I'll buy a cup of coffee for anybody who can show me side-by-side 24" x 36" prints made from a 5DIII and a D800 that, when made from the same tripod in the same studio and independently processed by the same (competent) person to maximize final print quality (not just the same recipe applied to both) for each which can be readily distinguished at standard viewing distances.
I can maybe see the D800 pulling away at 36" x 54", but still not at standard viewing distances. And, once again, if your bread and butter is 36" x 54" prints (or bigger) and the 5DIII format won't get you the gigs, you need medium format, not the D800.
shooting scenes with lots of dynamic range (IMO a huge difference here, this is the biggest difference)
In what real-world situation does the 5DIII have insufficient DR but the D800 does where you wouldn't be doing multiple exposures for HDR in the first place? And let's not forget that the 5DIII can do 6 FPS fire-and-forget several-shot brackets with a just couple button presses.
Once again, as I see it, if the 5DIII is inadequate, the answer isn't the D800 but something else entirely (in this case, a different technique).