I just got in the TC20E III teleconverter yesterday and made some shots with it today, using the 70-200 at 200, thus giving me 400 @ f/5.6.
I was pleased but not blown away. Even tripod-mounted it didn't resolve detail the way the 200 does natively. It's true I've just started working with the TC20E, and I didn't experiment -- I just grabbed a few shots.
And $460 is something my budget can handle, whereas $9000 (the 400 f/2.8) isn't.
I'm not much of a conspiracist, but it is not in Nikon's best interest to produce a 2x teleconverter that would produce results on par with 400mm 5.6 lens. They would rarely, if ever, sell a 400mm f/2.8 lens if they did (nor the 200-400mm f4, 300mm f/2.8, 300mm f/4 or 500mm f/4) as the prices would barely be justified.
That said, keep in mind your shutter speeds when shooting at 400mm. VR aside, you can't expect to get the same results at the same shutter speeds (unless they were very fast to begin with). As soon as you drop the TC2 on you'll need to double your shutter speed for the new focal length (when hand holding). So, when you compare a handheld shot at 200mm, f/2.8, 1/200sec., ISO 200 against the 400mm handheld equivalent your shutter best be at least 1/400 at f/5.6, ISO 200. Why, VR aside, a good rule of them is the slowest you should handhold is 1/focal length with 1/60sec. being the slowest regardless of focal length (your mileage may vary based on how steady you are). So, at 200mm, 1/200sec and at 400mm 1/400sec. Next, your f-stop has been double which means you lose 2 full stops in light. So, to truly make this a side by side comparison when handheld, you'll have to have enough light when shooting at 200mm f/2.8 that when you drop on the TC2 you'll have enough light eliminate any possibility of camera body shake. How much light? No less than 3 full stops. 1 full stop for the change shutter speed due to the change in focal length and 2 full stops for the change in the aperture. So, if your handheld shot at 400mm, f/5.6 is 1/600 of a second at ISO 200, then your identical shot with
identical light at 200mm would become f/2.8, ISO 200 at 1/4800sec!!! That's a ton of light!!!
Again, this is ignoring VR!
So, if you are hand holding your camera and you don't have that much light to begin with, you're pretty much guaranteed unsatisfactory results.
Of course, the best way to draw a comparison with and without the converter is to use a tripod (VR off) and take a picture of something that only moves during earthquakes. Then shutter speed is irrelevant.
After a fair comparison is made with out VR, turn it back on and check your results against those with it off. This will give you an idea of how successful you can shoot with VR and the amount of light needed.
When used properly, most teleconverter / lens combinations are a great low cost solution for getting extra reach. As it happens, all of Nikon's latest teleconverters work superbly on the latest 70-200mm. Of course they can't compete with a lens with twice the focal length that doesn't need a converter, but that would be an unfair comparison to begin with. The best measure of effectiveness of a teleconverter, when used properly, is if you are happy with the results. If you are, then the money was fully worth it. If not, then you are either getting paid to shoot or, IMHO, are likely a pixel pusher.