I realise you can't do a blind test for driving different cars, but one of us on this forum will have to be very good at driving to notice 1.6 inches difference. The BMW and the Audi are great cars in S, RS and M guise. 4 WD will give you an edge in wet or icy weather with the correct tyres and a good driver. However 4 WD will not help you brake faster... and 4 WD on ice with summer tyres doesn't help that much more in bends and anytime you are heading downhill.
In the real world there will be very little difference between the cars other than people's specific preferences. 4 WD helps many of us overcome our deficiencies when driving, we can apply acceleration more quickly knowing the 4WD will help us out. However most racecars that I know of are 2WD and are nearly all RWD... now surely we aren't trying to say they are all wrong? 2WD and 4WD are different experiences and generally have different styles of driving on a track. On a public road there's not a lot of difference between the 2.
For Audi to improve the handling it's not just about shifting the engine back 1.6 inches, it's about lightening the load which is simply too much at present. I'd quite like an RS and a couple of M's from the new range and a months pass to the Ring and a few other circuits to test them. All those in favour please sign up and I'll petition the manufacturers