The specs coming out of this rumor may not be eye-popping or drool inspiring...but rest assured the mk3 will dazzle with it's low-light/high ISO performance. It's not going to be a body you grab to shoot sports, and Canon hasn't intended to be.
Unless you're going to spend $6k-$7k for a body like the 1D-X you aren't going to get a "do-it-all" body. If you have a sub $3500 budget, you need to buy a second body like a 7D to be able to track your moving subject and have the burst speed to not miss as many shots. And the 7D is going to have Rebel quality ISO performance.
I wouldn't be surprised if AF was kept the same, but that wouldn't stop me from wanting to pick one up. If it gets 2 stop improvement in ISO over the mk2, that means clean shots at 6400, and would most likely be enough for people to upgrade, however its reasonable to assume that we will also see improvements in metering/dynamic range, perhaps some other minor things. But you definitely won't see a great AF system on a 5d body, not now, not ever....so please don't expect that. Save up for a 1D-X