I'm not so excited by this rumor. A lens like that, to have an IQ and an AF speed comparable to the Canon f/2.8, should be extremely big, heavy and expensive. I can't imagine Sigma producing a "monster" like that. Not even Canon, BTW. So we'd probably end up with yet another super fast lens that you can't use WO and that has an IQ far from stellar. What's more, I never liked Sigma lenses. All the samples I worked with didn't handle flare & ghosting well enough. I actually keep the 105 Macro in my equipment because of that, I like the effect. Maybe I always had bad copies, but I find it hard to believe.
I'd be much more interested in Canon producing a 24-105 (or even just a 24-85) f/2,8 with IS. Until then, the Tamron 24-70/2.8 VC is a good option, though I abhor the positioning of the focus ring... why the hell they put it there???