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Your wastegate is opening too soon, below factory spec. I would not dink around with an external boost controller, I would fix the wastegate properly! Assuming you have a Garrett turbo, you adjust the rod length from the canister to the valve. If you had a KKK there would be no rubber hose to tap in to so I'm assuming you don't have a KKK. The manual controller should not let boost build faster, you can achieve the SAME result by adjusting the wastegate directly.
I just checked specs for your car. Starting in 1992, MB lowered the boost spec to 0.75-0.85 bar, or roughly 11-12.5psi. So since yours is less than 10psi, your wastegate is opening too soon!
Tip: If your wastegate controller doesn't increase boost, or if you pinch off the hose and boost doesn't increase, there is not enough fuel to provide enough boost to actuate the wastegate. That means an ALDA adjustment is required. It is not "foolish" to adjust the ALDA, as the springs inside the IP weaken with time and the easiest cure is to adjust the ALDA *SLIGHTLY*. Of course the proper fix is to pull the IP and have it calibrated on a Bosch test bench, but most people don't do that. Mercedes even has a procedure to cure "sluggish acceleration with cold engine below 2000rpm" that specifies a 1/4 turn adjustment of the ALDA screw.
Regards,
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