I remember seeing waaay over 15 psi briefly as I pinched off the wastegate sensing hose once on a 167 diesel. The turbo is similar in these engines, the method of controlling the wastegate is different however. The Boost spec for the 603 is just a hair under 1 Bar max, or 11 - 12 maybe even 13 briefly. I wouldn't settle for less than that range under initial takeoff. How fast it comes up is important, maybe more indicative of turbo condition than the final resting pressure. If you're going to achieve 0-60 under 13 seconds the spool-up time is critical. I've come to know that these engines live by and for their turbo.
My sick turbo was just able to hit 7 psi, and the car was very sluggish. The worst sluggish 603 I drove was one that was for sale. It had to have a plugged trap or sick turbo or both, a 240D could have beat it!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!