That's true. People often forget that. Look at the drag race on Top gear between the R8 V10 and 911 turbo. First the 911 won it then the Audi won it.
There are so many factors to take into consideration. To get really accurate results each car should be taken around the track 2/3 times to see the best performance that can be squeezed out of it. There is no way that in every attempt the driver will manage to take every corner at the same speed and line. Also, some drivers may be better at driving the RWD M3 yet may not be getting the best out of the RS5's full performance.
That's why sometimes we have seen the M3 faster round the track and sometimes the RS5. I think if the Audi was lighter it would be a lot faster. You can see real potential in the car but weight really holds it back. Personally from day 1 I wanted to see a 4.0TT unit in the car for more torque.