Well, performance and suitability for conversion to R-134a is going to vary from model to model. MB used a bunch of different compressors (York, GM A6, GM R4, bunches of Nippondensos...) and just as many variables in the other components of the systems.

My old 123 300D could only produce 48F vent temps at highway speeds duing a Dallas summer. And that was with the recirculate function modified to use 100% recirculated air rather than the 80% the factory sets it up with! While 48F temps are just barely adequate, my opinion is that anything above 50F doesn't cut it. The car does not cool enough, and too little moisture is removed from the air - it has a muggy, humid feel to it. That's why I say R-134a doesn't work.