I think that because Sigma has such a small percentage of the DSLR market that their lenses' performance on Sigma bodies is irrelevant. Sigma isn't selling very many Sigma-mount lenses. Therefore, it only matters what a Sigma lens can do on the Canon and Nikon bodies that they are actually being sold for.

Knowing that a Sigma lens performs better on a Sigma body is quite irrelevant if you have a Canon or Nikon DSLR, at least from a consumer's perspective.

For what it's worth, I plan on using the Sigma 35mm f/1.4 Art and 50mm f/1.4 Art lenses as my primary tools to photograph an awards banquet tonight. I know their AF consistency isn't quite as good as some Canon lenses, but overall, they're good enough to be relied on in most situations. And when in doubt, using the center AF point (and recomposing) seems to be the best method for dealing with most of the AF issues I've experienced.