S6 and S8 motors are very very different, all the way down to bearings, rods, pistons, turbos, etc. The RS7 & S8 motor are supposed to be mechanically the same minus induction (dual TB vs single).
They are supposed to be very similar, with the S8 having the dual TB advantage. Only time will tell when we start seeing the RS7 hit the strip tuned and stock. The RS7 *should* be faster as it has a little less weight to tug around.
Wonder what de-catting will gain on these....