Just for the heck of it and to get double duty out of my Nikon V1 I use for digiscoping, I decided to test out the V1 with an old Nikkor 400mm f/3.5 manual focus lens with a matched Nikon TC-301 2X teleconverter. This put me out to around 2160 mm FOV with this rig. It turns out to work pretty well. Here's an example of a Belted Kingfisher I took with this combo. I still digiscope, but good results at this focal length and the light-gathering capacity and resolution of this rig is impressive.
Rick
That's a very fine shot of a Belted Kingfisher (tough subjects!). I'm very impressed with the resolution of the shot, it speaks well to both the 2x TC as well as the 400mm lens. 800mm is some reach, but you still had to be hiding somewhere closeby to pick up that Kingfisher!
Just so you know, technically speaking, you're only reaching 800mm f.l. with the 2x TC. I know, you did qualify it with "FOV," but the gain suggested by the "crop multiple" is a somewhat spurious -- not b/c you don't gain something by using a full format lens on a crop sensor, but b/c the technology is doing something that's not immediately apparent.
The reason crop bodies are affording some extra "resolution reach" is twofold: Photosite density & lens sweet spot.
The Nikon V's Cx sensor (a 2.4 crop Aptina) has 3.4 micron photosites (same as the 24MP Sony sensor in the D3200 & D5200). Having photosites that finely packed enables the sensor to resolve some very fine details from top-notch lenses. And it just so happens that your Nikkor 400 is an excellent lens & is sufficiently sharp to serve the sensor's 3.4u pixel pitch resolution. Even with a 2x TC, which understandably is going to soften the image by the same factor, that lens is still rendering fine on the 2.4x Cx crop sensor.
Here's something else surprising about using larger-format lenses on a crop medium: By only using the sharpest, brightest & contrast-iest part of the lens' image, you may even be gaining a half stop (or more) of brightness over the rated aperture of the 400mm. Since the 400mm Nikkor is very sharp in the center, and you are cropping at 2.4x, you are taking the absolute sharpest center of the image circle & projecting it on that Cx sensor's closely packed photosites.
The crop sensor is effectively *upscaling* the highly resolving center of the lens' image circle by token of its finer pixel pitch (about 1.4x finer than a full frame sensor's 4.9u pixel pitch).
I'm seeing a similar level of gain in relative brightness using medium-format long lenses on a D3200 ... the crop factor from 645-format film to a Dx sensor is 2.5x (close to your Cx sensor's 2.4x).
OK, so, since the Cx sensor's crop is 2.4x, 800 x 2.4 ='s 1920mm FOV. But at what point will the absolute resolution fall below this very simple metric: Lenses resolving to 55 lines/mm qualify as sharp (l/mm is the old school metric, new being line pairs/mm or 27.5 lp/mm). It would appear that your 400mm resolves a very sharp image in the dead center, somewhere in the neighborhood of 83 lines/mm (if you can crop to 150% & still look sharp), or perhaps 110 lines/mm (if you can pull off 200% crops).
The point being that a lesser lens will not lend as well to the resolving power of the crop sensor's 3.4u (micron) pixel pitch, and so the functional *gain* will be lowered. A superb lens might allow for deeper crop in post-processing (say, 200% crop), while a decent lens might only afford 150% crop. A 3rd-rate lens will drive you nuts if viewing 24MP images on Flikr, but it'll be OK if printed on 3x5 glossy, but at a 50% crop (or -66% crop )

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So if 150% crops are coming out OK from the 800mm effective focal length, then your effective reach is 1200mm. If 200% crops, then 1600mm (tack sharp). If a mediocre lens were used (say a Samyang 100-500/5.6 zoom full of fungus like the mushroom bazooka I got for $20 at a yard sale) then the effective crop is going to be 50% (it won't look sharp unless I shrink the image .. in other words, the effective reach reduced by a soft lens (well, we knew this...)