This weekend I thoroughly flushed the system: evaporator, condensor, and hoses. I first flushed with air, then with paint thinner, then with a/c flush, then with brake cleaner, and finally with air.

I put 6 oz of ROC oil (universal refrigerant compressor oil) into the system:
3 oz into the compressor
2 oz into the condenser
1 oz into the evaporator.

With a new receiver/dryer, I evacuated the system for 3 hours.
I charged the system with 12 oz of Duracool (16.5 oz recommended).

I also fixed the blend door that was not closing.

At 103 degrees and 70 mph, the middle vent temperature was 59 degrees. This was the best performance so far. My 89 Pontiac Grand Prix would put out about 52 degrees under these conditions. Considering the undersized A/C system in the Mercedes, I think my A/C is the best it can get.

At this point, to get any cooler, I would have to change the condenser to one with double the capacity like the one used on a 380 SE.