Have you checked your vaccuum lines going to the turbo? A leaking vaccuum line can cause your turbo to engage later in the RPM band.

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Ed
-1984 Mercedes 190D 2.2 5-speed gray market(bought@30,000 miles) (Sold back to original owner@170,000 miles)
-1999 Mercedes E300DT (245,000 miles)
-1999 Mercedes S500 Grand Edition (80,000 miles)