A very intriguing thread !
Browsed some docs and found a difference in where the place the measurement takes place:
-between fuel pump and fuel main filter(w116 om617.95x)
-between fuel main filter and IP.(w115,w123)
Incorrect translation maybe?
The values given are the same.However,other manuals suggest that a clean fuel filter restriction may amount to 0,2-0,3 bar difference.
No document decribes testing the fuel delivery pressure under load-only at idle and high idle(3k rpm)..

I`ve applied pressurized air (>2 bar)to the outlet port of the fuel pump,expecting at least some air loss at the roller follower side-nothing(but no fuel dilution in the oil sump either)...just slight air leak from the inlet valve(I don`t expect them to seal completely)...and one can feel how easier is to push the rear plunger when the main spring is compressed. Think it`s good to change the main spring after decades of operation. ..Has anyone checked the delivery end pressure?