I've been thinking about the possibility of the AWD Model S and what that would entail speed wise and battery wise for a while now, and I think I've come up with something that makes sense and is consistent with everything that we know so far.
First, in the fourth quarter '13 earnings call Elon said that the cadence of Model S updates would be a substantially redone model every 6 years or so with an intermediate update every three years. This would mean that a '15 Model S would be a perfect time for such an intermediate update, while the true next generation Model S would come in '18 near the release of the Gen 3. Therefore, I think it is safe to assume that with all the rumors of AWD floating around, as well as other things like improve seat comfort as an option, that the '15 model will indeed have AWD as at least an option and will represent this intermediate update.
As for the speed of the Performance AWD, a previous poster was correct in saying that currently the Model S is limited in total horsepower by the maximum power output of the battery. Therefore a larger battery would be necessary for a significantly faster AWD option. This does not mean that Tesla will need a pack with physically more cells however, as battery technology has progressed significantly since the original Model S was released. As we know the current Model S uses Panasonic 3.1 Ah, 3.6V 18650 cells in the 85 kWH model. However, Panasonic now should have 4.0 Ah, 3.4V 18650 cells available. You can find more information on these batteries
here if you can speak Japanese. This would mean that a drop in replacement would yield a battery pack of approximately 105 kWH which lends credence to the battery pack in the 100-110 kWh range. This in turn could lead to the higher power necessary to drive a significantly more powerful AWD Model S.
Seeking alpha did an analysis on such a vehicle, albeit assuming that the increase in performance could be accomplished with a 85 kWH pack. They could that the Model S was initially designed to accept the drive unit from the electric RAV as a front drive unit in conjugation with the 362hp S85 motor as the rear drive unit. They found that such a configuration would have a 0-60 time of 3.2 seconds, and around 516hp.
It's also worth noting that the Model X site says that there will be 60kWh and 85kWh versions of the models X available with AWD in addition to a performance model. This necessarily means that Tesla must be working on 3 separate AWD drive trains, irrespective of the capacity of the performance model battery pack. Also, at the Model X introduction event Elon said that the non performance AWD 85kWH model would have a 0-60 time of ~4.4s, on par with that of the current performance P85. This makes me think that the planned 85AWD drive train uses the S60 RWD motor as the rear drive unit. Assuming that all of the models have the same front/rear power split this gives a total power output of ~430 hp, or roughly the same as the current P85. Taking the same analogy to the 60AWD drive train it seems likely that it may use the original roadster motor as the rear drive unit for a total power output of ~363 hp or around the same as the current S85.
Not that this is all just an first approximation based on what has been said by Tesla and what seems technically reasonable. There may be all kinds of market forces that prevent this from happening.
So to summarize the best case scenario on the AWD front for the Model S would be that AWD is offered as standard or as an option on all capacity levels, and that the performance model may get a higher capacity battery to run the more powerful drive train. Giving us a ~1s reduction of 0-60 time on all AWD models as compared to RWD models.
P110 AWD 516hp 3.2s 0-60
S85 AWD 430hp 4.2s 0-60
S60 AWD 363hp 5.2s 0-60
