Sorry but the rs4 is not the lambo gallardo engine! Lol. The R8 has a detuned v10 that is the lambo engine but not the old rs4 (or any car except the R8).
Carbon build up is a big deal because it robs a ton of power. Maybe is not a hassle but to have power sucked away when the car already put down less power to the wheels than really stated. Usually was putting down 310-320 at the high end on countless dyno's. Pretty widely accepted this thing was putting down less than what audi stated. Couple that with another hit of power from carbon loss and the fact it weighs 4k pounds, the m3 simply performs rapage on the rs4. Its not even close in a straight line and around corners.
Heck the m3 still beats the relatively newer rs5 in straights and turns. Despite 450 hp, the rs5 is 4k poudns and has drivetrain loss through AWD