The 103 engine is very robust, but you should not demand full load and revs of ANY engine until it is fully warmed up. Remember, Germans drive these cars flat out on the autobahns!
When I need max performance from my stick shift '88 190E 2.6 I shift just short of the 6650 rev limiter, even though the redline on the tach is 6200. It makes useable power all the way to the limiter.
If you need maximum acceleration from a dead strop pull the shifter to low, which will force it to start in first gear instead of second. This will considerably improve initial acceleration by avoiding the downshift lag when you floor the throttle in drive. In drive your car starts in second gear. You can also improve acceleration by shifting manually and holding the gear longer by upshifting just short of the rev limiter.
Car and Driver's first test of a '86 300E (a stick shift) yielded 0-60 in 7.5 seconds. Though the 103 engine is torque shy, if you use the sweet spot in the power curve from 5000 to the rev limiter, they are fairly quick.