I am 99% that the engine in RS5 will be V8 4.2 FSI 420 HP, also known from RS4 B7. But I wouldn't mind some extra power...M3 will be lighter than RS5 and RS5 will have AWD, which means that in straight line, we probably know who's the winner. But we also know that driving is not just a straight line and that's where Audi will be better than M3. Just guessing.