Joe - I don't know if this is going to answer your question or not but I have been researching on how to improve the efficiency of my a/c on the 1985 300D that has been converted. I was talking to a guy from a company that builds/supplies a/c systems to old cars (VintageAir) and he said that I should be able to get 42dF air out of my converted 300D with the original components. He said the procedure is to evacuate/vacuum the system, put a big shop fan blowing into the radiator area, add 1# of 134 and then add 1 oz at a time until the temp drops to 42 dF out of the vent. Yesterday afternoon I stopped by my a/c techs shop and asked him to put his gauges on it. He did and the low pressure would drop down below 40 psi BUT the whole time it was swinging back up to around 50. He said that this is an indication that the expansion valve is not "settling" down after which time I ordered a new expansion valve (thanks fastlane). Anyway, in discussing how to improve the system he said that increasing the condenser size will probably not help a whole lot. I am working with him to get the new expansion valve in and will see what happens then. Currently, it only gets down to around 65 dF out of the vent - this is with 95dF outside air temp and 100% recirc. I will keep ya'll posted.
I would assume that you should be able to measure condenser efficiency by looking at the pressure/temp in and out of the unit and then going through some calculations on the a/c curves. It has been along time since I have done this. You may want to try to find a good local a/c tech that can help you in your town.

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Jim