After a few years of very amateur shooting BIF from the Bigma 50-500mm F4/6.3 (MkI) then to the Tamron 150-600mm A011 then the excellent A022/G2 version and the AF-S 500mm ED F4D, I just wanted to pass some comments.
The G2 and the D500 seem like the perfect combination and it almost is.....at least for "most" BIF shooting .......the caveats are you need good light, preferably sunny conditions, typical settings are M mode 1/1600 @ F8 at around 250-500 ISO, given that, in most situations there are few problems and one can expect very decent images.
The problem comes when shooting dark birds such as vultures and dark brown Ibis for example, for the most part you are shooting from a lower viewpoint than the bird and it's underside is in shadow. Dark brown in shadow reflects very little light on to the sensor, looking at PS's histogram figures from 0-255, on the underside of a vulture, I'm getting numbers like 10-30, hopelessly noisy.
.....a light gathering light war ensues, juggling ISOs / shutter speeds and aperture, in the end you lose the war!
The D750 goes some way to solving the problem in the same conditions, producing much less noisy images which in turn gives sharper images, (using the whole G2's field of coverage), in fact it seems that in theses testing situations the FF sensor does way better than the "claimed noise differences" of around a stop, in these situations I think it's closer to 2 stops of improvement........but you have lost the reach advantage, but, the battle is not lost.
The other option is to climb these cliffs with 7 Kgs of backpack (4.2 Kgs of AF-S 500mm+ D500+ water+ bits and bobs....heavy when you are climbing a slope of 35-45%)
.......... but you can shoot the D500 at F4 (750mm equivalent) giving you 2 extra stops of light.
The same lens on the D750 gives a three stop advantage, but with only 500mm of reach.
So for dark BIF shooting unfortunately the G2 and D500 just can't cut it and it's no surprise that pro shooters still want their very low noise D4s/ D5s and their heavy and expensive pro lenses.......until sensors can improve by 2 stops the old rules of the game remain, the bigger the better.....both for sensors and lenses!