Having owned one in the past, two things I would verify.

1. Are the cats plugged? That will kill all top end power, and the engine will just dwindle in that range and have not much output.

2. I had a bad o2 sensor in mine, and it made it feel quite sluggish.

Im assuming your fuel pressures are ok.

As for the M103's being slow, I disagree. They definetely utilize all 177 ponies to the max. In a drag race with my friends 88 300E vs my 92 400E, he killed me to 50mph, when I just started to pul ahead. Properly running, they are peppy.

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K