I wonder if this is due to using Al in the front and Steel in the back? Any idea how much the whole car weight is affected? Between shaving weight (presumably, most of this occurred in the forward half) and redoing the drivetrain layout, one would have to have some elevated hopes for the handling.
They're kind of vague on the exact model, but in the latest Motor Trend they give the stats for the A5/S5 as 3,770 lbs with a 58/42% weight distribution.
So, not exactly light-weight, and still pretty nose heavy.
From what I've read, handling is better, but hardly a night-and-day transformation from the old car. Giving the AWD more of a rear bias probably has as much to do with the improvement as changing the weight distribution by a percent or two.