I agree. I doubt the AMG 4.3L V-8 was suffering from excessive backpressure. The AMG exhaust system was carefully engineered to work with the engine and intake system. Things like the camshaft profiles, timing and intake runner design were all carefully chosen to work together. Do you really think they left a bunch of HP on the table? Not likely.
Your aftermarket exhaust has probably resulted in a drop in midrange power, and possibly peak power as well.
1998 C230 "Black Betty"