Im a 2000 M5 owner and have a cousin who has a stage II S4 (325 BHP). I can tell you that the S4 Keeps up with the M5 till i almost shift to third. The S4 is fast and has alot of torque thanks to the twin turbo. But flooring it about 80 Mp/h, the M5 just pulls away and hard. The S4 couldn't handle the M5's V8 pulling power, it is just huge espacially in second and up wards. You got to remember that the horsepower difference is about 80 hp. So no matter what, that will keep the M5 pulling beyond the Audi's capabilities. The S4 is light and has alot of torque. The S4 is fast though for most cars including a porsche carrera that lost to it the other day:) Well i can't say much about the M5 you drove. Maybe the engine hasn't broken in yet. The engine is very tight and you can say slow till it reaches about 5000 miles. Thats when you start to feel its power. I mean my car has 12000 miles and hasn't reached its peak performance yet!!