Anyone that cares for their engine and actually opens the hood between oil changes won't have much chance of having an overboost situation. It's the kind of people that just drive and ignore general maintenance for 10 years that MB included protection.

Blow into the wastegate tube with a (regulated to 15psi) air nozzle, it should hold air on it's own and you should hear it "clunk" closed when you let the air out. If you are brave, you can take off the back panel on the T3 and look at the valve and seat. Chances are, though, the nuts are rust welded to the turbo at this age. The KKK's have it easy (again ), you can remove the entire wastegate actuator and valve since it's just held in place by 3 bolts. I know mine has a cracked valve seat, but I doubt it's hurting performance enough to think about replacing the turbine housing.