A free-flowing exhaust can bring gains in horsepower for diesel engines. Any exhaust that causes a restriction, or backpressure to be created will also decrease efficiency and horsepower. It all depends on how well the exhaust system was designed. Generally there should be no more than 0.5-1psi backpressure in the exhaust system, measured at WOT, under load (accelerating), with the psi reading taken a few inches after the turbo or exhaust manifold. MB designers likely did a good job creating a free-flowing efficient system, so improvement may be difficult. My experience has been with turbodiesel trucks. Changing from a stock system that generated 4-5psi backpressure to a custom performance setup, producing 0.5psi, the difference is noticeable in acceleration and fuel mileage. Generally a custom exhaust with pipe diameter increased 1.5-2 times the stock figure, and a free-flowing or no muffler will produce the desired result. Turbodiesels are surprisingly quiet with no muffler as the turbo does an excellent job of smoothing and damping the exhaust pulses. If you are really serious I recommend testing the backpressure of your current system and making changes as neccessary. RT