How much boost are you making? You certain the waste-gate isn't opening early? The 603 has a weird power band, the sweet spot seems to be between 2000-4000 RPM. If you're too far down in the rev range, hit the downshift button and it should light a fire under it. If you're close to or above 4000 RPM, back off the pedal and let it upshift.

My experience with the 603 is that it is NOT a rocket-ship, however it is very capable. Spunky around town, competent on the highway. Passing someone at 60+ may take a while for a run-up... Make sure your kick down switch is working. The 603 likes RPM, especially when trying to accelerate. Too much RPM and it'll accelerate slower than if you let it upshift and stay in the peak torque band.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]