The only R8 quattro that I've driven in the game was during the first race. Scratch that I also drove the Nightmare VIP car when it was gifted to me but I've never set out to toon one for A or S class. I liked the way it drove, typical understeer for an AWD but it seemed a bit more managable than some of the Evo's I've driven in game. I'd bet it would be one of the easier cars to setup but I could be completely wrong.
If you are having trouble with a build or a specific area of tooning the car feel free to post what you already have and someone will undoubtedly have a look and offer an opinion on what could help make it even better.