1982 Mercedes 300sd air conditioner question

1982 Mercedes 300SD turbo 5 cyl diesel mileage: 92,000. I have just purchased this car and I had heard the air conditioner's on these cars are not the greatest! As a matter of fact, I do see an awful lot of these cars driving around with the windows open in the summer. I live in Florida and I can't imagine these other drivers 'just like fresh air'. I had the Freon 'topped off', it didn't require much (a mere 2+ ounces) and it does not seem to have helped. The best I can figure, from asking questions of local mechanics, is the evaporator is simply 'too small'. The air conditioner barely cools, do I have to live with this or are there some suggestions you can make to improve the cooling capabilities?