Don't know whether Quattro is better than Xdrive..
In fact, if this test is relevant to real life (would presume so) then if anything Xdrive has a slight edge..
What I do notice is that when four wheel drive cars go out in the ditch here in Sweden during the winter, the damage is usually quite high - indicative of a too high speed for the road conditions.
My old E46 with rear wheels driven would squirm and indicate quite early on that grip was in short supply. The 540i is like a steam roller... until you start braking and realise how little grip there actually is.
When it comes to braking then there is no difference between a two or four-wheel driven car...