Hi jason...
I have thought all along that reducing some back pressure by getting some of these manufacturers replacement silencer/mufflers or a cat back system starting from the resonator. But i read a recent article in MAX magazine. where they did stage by stage bolt on improvements on a VW normally aspired A4. The intake which was a cold air system from K&N and the Chip added a little bit of HP and Torque. About 3-4 HP/Torque. But once they added the cat back system, the reduced back pressure gave them negative return on their acceleration but helped in the higher RPM range. So they lost about 1-1.5 HP/torque but they gained more aggressive sound and looks.

so, when looking for a system for my car, CLK430, i want to be very careful. From my research, i think its either i go with the entire exhaust system from an expensive tuner like Carlsson, or do nothing at all. The price for performance gain wouldn't be worth it if i just the cat back or muffler alone. then comes the next reality, if i want the maximum performance out of my CLK, i should just trade it in and get the CLK55!! (too expensive for me!! )

take care and thanks for the reply...