Maybe I'm way off base here because I don't know the MB Kompressor system all that well. (All my experience has be NA) but ...
Doesn't the Kompressor system kick in around 1500 rpms? Without either some sort of bypass system in the exhaust, I could be wrong but, an open or too large an exhaust would just kill low rpm performance.
When the Kompressor engages, does it engage at full boost? If not, you still need properly tuned exhaust dynamics to not kill performance ... especially torque, which for me personally is more fun than horsepower.
I'll let the experts confer here, I'm just scratching the ole noodle, but I always thought you needed a proper balance of back pressure for the greatest velocity and scavanging with this type of SC setup. (And maybe I am putting the horse before the cart because the velocity/scavenging is what creates the backpressure) I know with a Roots you can just run open headers and be done with it.