Originally Posted by swamp2
A quick check indicates Audis quattro system has a permanent 40/60 split F/R whereas the Carrera system can vary the split between 40/60 to 5/95. The GT-R varies from 50/50 to 2/98 and it makes adjustments every 20 ms! (Side note BMWs xDrive system can vary between 0/100 and 100/0 but I can not find how often it adjusts). Hence the characteristically high drive train loss we see on the RS4 and hence also the close acceleration times you have found comparing C4 vs C4S.
Actually swamp this is technical wrong, Audi's quattro split is indeed 40/60 but it's can vary as much as 25% in either direction (65/35 or 15/85) and it performs this in a time compatible to the Nissan. As for the X-Drive, yes it can vary fully in either direction but it is quite dimwited in comparison and is much more inclined to spin the wheels for Quattro will.
I can't comment on whether Quattro loses more power than either Porsche's setup or Nissan's but I can't believe it's any worse then either because Autocar tested both the M3 and RS4, their split times at each speed segament right up to 150mph only place the two cars 0.3s apart and as both have identical power outputs and their torque figures on the most part are also very close one must assume that Quattro doesn't lose that much power at all. Also you have to remember that the RS4 weighs over 100kgs more in that test which is surely another disadvantage in terms to acceleration.