I'm sure we have all watched videos of a 4 wheel drive car leaving a 2 wheel drive car sitting on snow. I would love to see the exact same test carried out with the same make and model of car, fitted with the same tyres, trying to stop. Like you said the quattro cannot defy the laws of physics. In theory the extra weight would mean that the car fitted with 4 wheel drive will have a longer braking distance in all conditions. This would be most noticeable on snow and ice.