It is easier to achieve peace in the Middle East than it is to get a casual Audi fan to understand that the horsepower ratings of both of these cars could not be more misleading. This is one of those times where traditional BMW strengths are exemplified. What are those strengths? Balance, less weight on the nose, and a more efficient drivetrain sending its power to the rear wheels.
Because BMW does not need Quattro crutches BMW's claimed 425 horsepower actually comes out to 427 horsepower at the rear wheels. Yes, the M4 is heavily underrated but it also benefits from not having to send power to all four wheels and saves weight by not having an all wheel drive system. The RS5 has roughly 350 wheel horsepower depending on the dyno used. Audi's higher crank horsepower rating means absolutely nothing in the real world and this is why Autocar stating that the RS5 has more power is wrong. It has less power. A lot less power (and torque). These people at Autocar get paid for these incorrect statements?
Power is only one part of the equation. The traction (not handling or grip in corners) advantage is with the Audi. However, once moving at speed the extra weight on the nose from the all wheel drive system is not exactly a benefit on the track. Understeer? That is the Audi trademark over the years and it is present on the RS5. It definitely can not adjust its rear with the throttle to the degree the BMW can. This is all reflected in its laptime of 1:12.20.
The M4 is said to be more fun to drive and it is. It takes more skill to extract its performance but it is more entertaining. It is the driver's car and this is the area BMW has traditionally always beat Audi although Audi has improved tremendously. Sure, your Quattro will save you during a blizzard but nobody goes on the racetrack during a blizzard. In the dry as in this comparison where maximum performance can be extracted the M4 crushes the RS5 with a 1:09.90 laptime.
2.3 seconds may not sound huge but on a track this short that is a lifetime. The M4 is simply lighter, faster, more powerful, and the higher performance car. The traditional BMW aspects trump Audi's traditional aspects in the performance area as they have for long time when the M3/M4 and RS4/RS5 are concerned. Sometimes the more things change, the more they stay the same.
Audi fans will be quick to point out the RS5 is in need of update. They are right. It is desperately in need of an update and a hell of an update at that or else this will continue to be embarrassingly one sided as it has been for generations now. The benchmark is set. The benchmark is the M4.