If your 300SDL is running at 5500 rpm then PLEASE drive with retraint OR hope that the tach is not reading correctly (they are adjustable - and aren't completely stalwart). If the clock's been repaired, the tach is likely to have been disturbed (don't ask how I know this ;-).

The design limit for the wastegate is 0.95 bar or ~14 psi. Many OM617.95s don't improve much as the boost is raised above about 10 psi. (and I'm NOT exactly sure why) while the OM60x turbo engine are more responsive as boost rises. The overboost switch is set for 1.1 bar and this is the simply a safety device in case the wastegate fails. You CAN (with a lot of luck) run an engine for years with the boost well in excess of the design limits and with the ALDA spooled up if you don't ever push it for very long, but some large number of such engines WILL fail without ANY warning within minutes in a full load situation. When they let go, a rebuild is seldom possible or economically practical.

Marshall