I believe that your car is running VERY Lean at a duty cycle of 78%, not richer.
The MB manual clearly states:
"leaner = high ON/Off Ratio"
Since you find no difference in performance, may be the closed loop is compensating with enough fuel such that the performance is not affected, since the compensation goes from 10% to 80% (according to the graph below). I do not know how much this stresses the system.
Experts like stevebfl can give a better explanation.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car