My '83 380sel had already been converted to r-134a before I bought it. Performance was Ok, not great. On a typical summer day the air coming out of the vents would be around 50 degrees fahrenheit when the car was moving, a few degrees higher is stopped. I converted to Dura-Cool,
http://www.duracool.com/, one of the hydrocarbon refrigerants, and it was better, 42-45 when moving, about 50-54 when stopped.
There's a lot of controversery regarding the hydrocarbon refrigerants, but my experience has been that they cost less, work better and are easier to use as they don't require an oil change. If you go to r-134a from r-12 you'll have to flush the mineral oil out of the system and replace it with pag or ester oil. The hydrocarbon refrigerants are OK with any of those oils. In any case it's a good idea to replace the receiver/dryer.