Because all FF lenses perform better on crop bodies which take advantage of the sweet spot
That's true, but the FF optical performance of the sigma is not much in question. Its border resolution is as good or better than the Canon EF 50mm f/1.2 USM L (which itself is not very good in the border resolution department; see photozone.de, or the review by dpreview.com). The problem with the sigma is its AF. And I cannot see how AF issues would be less on a crop body.
For equivalent framing to use of the lens on FF, the depth of field on the crop body would be deeper. So, f tihere are AF issues, they would be masked on a crop body by the deeper DoF.
Hehe, I think we've covered this topic before. Actually, AF issues get worse, if anything, with crop. I'm not going through the crop vs. FF lens/image equivalence thing again, but will merely give you an example that will make you immediately understand why. Imagine you have a focus error with a FF camera, such that the de-focused resolution is 1/160 of the image width (0.6%). Then the equivalent image in a crop camera, would be to crop that exact FF image with a factor 1.6x. The AF error would now be 1/100 (1%) of the image size - which is clearly worse. Resolution in terms of resolved minimum angle would of course be the same (the crop image is, after all, the central part of the same image as the FF).