Eric,

Actually I have had good performance from my 123 and 124 cars with R12 in stop light driving. Everything has to be perfect though. There is not much extra capacity, that's why they don't convert well to 134 without more condensor.

Compressor efficiency has nothing to do with a/c effectiveness. If you have a more efficient compressor it will just cost less drag on the engine.

The controlling element in a/c efficiency is the condensor, providing of course that the compressor has adequate capacity which they all do.

Leathermang,

A cover like you describe should be easy to find from a German aftermarket supplier. They are quite popular in Germany for the Winter to keep the snow off the glass.

Have a great day,