Just my guess/thought. I've always felt that AWD alone would knock off .5 secs to 60 mph. Traction limits mean everything when you get under 4 secs. All it should take is a 200 hp motor driving the front wheels - I think the power from the batteries is already there.
When concept EVs go AWD, they almost always do it with a second motor (or 4th motor) since the motor is cheap, light and small. Given how this car is sold at least 50% to people focused on performance, it is a no brainer that they do AWD and bump the invertor up a little bit. A no brainer. The AWD will absolutely be a significantly faster car to 60 mph.
As far as timing, it should be purely a marketing decision. Do you want the even faster S to come out first or the Model X to come out first? I would split them by 3-6 months at least to keep the buzz going. It really is a production capability thing to decide which to do first. Given that we are approaching a year with the Model S (or have we hit it?), the AWD should be ready now. They are working on Europe first.