that I would use FX rather than back up or resort to the long odds of quantum mechanics by shooting a 300/2.8 unless I had a very specific reason to do so.
I would write-off anything inside of 16 ft rather than switch to a 300/2.8 hoping something would land somewhere between 6 and 16 feet. I believe FX and 700mm is a statistical winner in that regard.
(actually, the better I get at using my Sibley bird app the more of a problem this distance thing becomes but that's life, and part of the art of using that app)
As far as the rest, I'm not exactly sure what you are trying to say. I don't think that hard about it. I want the biggest piece of glass I can afford and can carry. I want an FX sensor with resolution at least as good as the D300 in terms of sensor density. Any more density is a bonus.
The D800 fits the bill. As far as 1.2x crop factors... I don't think about it or worry about. I just worry about shooting as many birds of interest as I can that land within some sort of shoot-able distance.
A 1x crop factor would be best, then a 1.1x then a 1.2x and so on. I want to fill the frame to the extent I can get a workable composition. With FX it is unlikely a small bird would create a problem on FX but larger waders and etc. certainly could.
I don't worry about doing this and that and then opening up 1/3 stop. I don't pick the working distance, the bird does. I don't pick the light; the bird does. When the bird perches in my shooting zone I do what I need to do in order to try to shoot it. I keep things simple in the field because what I do is complicated enough.
I suspect I have far more interest in shooting documentary images than you. That is why we are doing very different things. If a bird lands where I can make art, I try to make art. If he doesn't and he's a bird of interest I get whatever shot I can get.
I often spend days just trying to get one documentary shot of one bird, but in that case it is a very rare bird (at least locally). I spend a lot of time chasing rare birds. In this age of eBird it is actually easier to chase rare birds than more common birds.
I do not believe I am the "typical bird photographer" here. I started out that way but that is not where I have evolved the past few years.