I think that you will find that some of the fastest track cars are actually 2wd. Not that this is what the discussion is about. I would just rather have the M5 than an RS4 thats all. 
Why is four wheel drive banned in Formula 1.
Why is four wheel drive banned in the BTCC?
Why is four wheel drive banned in the DTM?
. . . . .
Think I am going somewhere!

The ONLY reason that some of these 'track cars' happen to be rear wheel drive is simply because 4wd is banned. Look what happened when Audi entered all the domestic Touring Car Championships with their quattro Audi A4 race car - it literally thrashed every single championship, including the (then) very highly rated rear wheel drive BMWs - and then the following season, 4wd was banned!

[/quote]
I think we are drifting off topic here slightly.... the M5 would still hand the RS4 its a$$ on a plate. 500bhp isnt exactly alot to put through a RWD.