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I realize no one will have this yet but I would be keen some samples of the new AF-S teleconverter TC-14E III taken with the amazing 80-400mm f/4.5-5.6G ED vrmy gut feeling is, you might see an improvement, over cropping , on a D4 but not on a D800E
I will say this again because I'm serious. The best teleconverter if you shoot full-frame is to buy a D7100. It's a 1.5x optically perfect teleconverter that loses you no stops of light. Think about it.
Modern TC's (any current Nikon offering) form part of a complex optical formula which includes its own corrections.
This is why the tc-14eII has been measured at around 5% loss of resolution behind lenses like the 70-200/2.8, where a simple perfect magnifier would lose about 30%.