If your turbo has an external hose to the wastegate, get a longer hose and hook the wastegate up to a post-IC pressure source.
The overboost protection solenoid kicks in at 17psi or so. From what the Finns say, the stock turbo gets pretty inefficient if you go much higher than that, so you may as well stick to 15-17psi post IC.

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'79 300SD W116
'86 190E 2.3-16V W201
'92 300D 2.5 W124
'99 E320 4MATIC S210