And were made with the idea that if someone is thinking they might want to buy a D500 to get the "wider" zoom range of 180 degrees to 110 degrees that this lens provides on a DX camera, which is similar to the Tokina 10-17mm lens, that they could get the same zoom range by switching their FX camera to the DX crop mode and lose very little in comparison to a DX only camera.
I would disagree on the point of "losing very little" between D800/D810 in FX mode and the D500. This is of course based on shooting all three cameras underwater a fair bit now! However, that is a discussion that we are best having somewhere else.
I would absolutely agree that there will be no vignetting or port shade in the image with the D800/D810 and this lens at 15mm. Equally I would tend to agree that the optical quality on this lens is much superior to that of the Tokina 10-17mm. This is especially true if the Tokina is combined with a TC on an high resolution FX camera like the D810. Build quality seems also to be in a different league. Most of us have had at least one Tokina come apart on us!
Strictly technically, the Sigma does actually focus closer than this lens (by 1cm). I note that Nikon list the FOV of the lens at 175° at 15mm on FX. The Sigma has an FOV of 180°. That 5° difference in FOV probably explains the lack of vignette.
The D810 images above were with a Seacam Compact port (170mm). I have used the Sigma with this port a lot and there is no vignette at any aperture. It is amazing to think that 10mm of difference in diameter produces such a difference. It is a salutory tale for anyone considering dome port choices.