OP here. I have an update, for whatever it's worth. Went ahead and purchased the 50A from B&H. Not surprisingly, it has AF issues. It was front-focusing right out of the box, and I used Reikan Focal to adjust it at 4 different shooting distances and then configured using the Sigma dock. Adjustments were + 15, +15, +17, +18. It's better now, but it is still inconsistent and erratic in ways that can't be fixed with calibration. (Note: I do use non-center AF points on my 5DIII quite a bit, so perhaps this is part of the problem. I've found that Sigma lenses don't always play well with non-center points.)
But even if the AF was perfect, I would still consider returning it, and here's why: the thing is big and heavy. It feels much heavier than the 35A, which I don't mind so much. But given that my two main lenses are the 24-70 II and 70-200 IS, I really want the 50mm to be lightweight and more compact.
I've owned both the 50L and the 50/1.4. I must have had a great copy of the 50/1.4, because I just went back and looked through my LR catalog and found that I had almost as many great keepers from the 1.4 as from the L. And my copy of the 50/1.4 was fairly sharp even wide open. I now find myself wishing I hadn't sold it.
I'm going to return the 50A and either get another 50/1.4, or just stick with the 35A and wait to see what Canon comes out with. I'm not in a huge hurry and can live without a 50mm prime, since the 24-70 II is such a phenomenal lens.