...why old Benz tend to outrace their catalog data easily:

a.) The speedometer usually is a little optimistic. They are usually set 7% plus.

b.) Mercedes tends to post rather conservative figures. Mercedes owners used to be pretty conservative, a lot of the drivers were elderly people who did drive rather slow. But even these slow engines should be able to reach top speed on the dial, so that the customers didn't worry.

Fast running diesels might also have a problem with compression. Diesels are different from gasoline engine in that respect. Less compression means they can rev higher. Better top-speed, less torque.

Kind regards
Eberhard