The chart below shows an approximation for how the stock B5 S4 intake system compares against an aftermarket alternative.  The various system components were tested separately, so the composite results shown below do not accurately depict how all of the parts would measure if operating on the automobile.

Additionally, when on the vehicle the operating pressure would in many cases be much higher, which would cause the pressure losses to be greater.  Still, the chart gives some indication to the scale of improvement that can be obtained through the use of aftermarket or alternative parts, at the tested airflow of 500 CFM.

The legend shows the different configuration tested.  To the left of the slash ( / ) is the stock configuration and to the right is the modified and/or aftermarket configuration.