I've got a 7D, and I've used a 5D2, all i can say is:
1 - yes (of course, that depends on your definition of "significant"). Low light especially, and even normal light, you will see a lot less noise. But you'll lose your 10-22mm (FF-equiv of 16-35), so you'll either need to buy a 16-35 (or 17-40) for the FF, or live with 24mm as your widest.

2 - yes, AF. 7D's AF is great, especially for sports and bids and such, and tracking across multiple points. I've heard that the 6D is good in low light on the centre-point, but never used it. So it depends more on what you shoot, birds in the sun or things at night?
Also, inbuilt flash, you may or may not use, and may or may not miss. I normally use my 430EX on-camera, but when it's not, having the ability to use the inbuilt flash as a trigger when the 430 is elsewhere can be invaluable (like in studios and such). Or just buy a long cord, or 580/600 to use as master, or another system like pocketwizard.

3 - That's going to start an argument, but seemingly the 6D isn't so bad when compared to the 5D2. But it is more expensive. I'll leave that to people who've used both.