I doubt there would be any change at all. From what I understand from my ownership of a Subaru, the center differential varies the amount of torque between front and rear. Whether it’s 100% front if the fuse is disengaged or 50% each way as needed for example is of little consequence; your mileage won’t change. It doesn’t change in use when on a straight level road, as much as 90% front to 10% rear may be the norm. If it did significantly improve mileage, don’t you think they would give you a lock out feature, like truck based and the very early Subarus had ?
Besides, in all of my 3 awd cars, tire mileage was significantly better than fwd alone. What you may “think” you save on one end, you definitely loose on another.