One cheap, relatively easy retrofit that would nicely complement a parallel flow condensor would be a heavy duty electric fan setup. As Larry said, low speed cooling problems are related to not having enough refrigerant, which is caused by a lack of airflow through the condensor at low speeds. One way to combat this is a serious electric pusher fan, or even two. I'm thinking of my old Saab 900 that had a dual fan setup where both would run when the a/c was on. I don't know how efficient the stock w123 fan is, it doesn't seem all that bad, but fitting the largest, fastest electric fan, or some dual fan setup, would probably be even more efficient. And then hook it up so you can turn it on manually if necessary.

Its hot here in DC in the summer, and I find that my electric fan will run almost all the time in traffic, but I don't get really great cooling if I'm stuck in traffic. But remember, I have r-134a in my car, so my vent temps are really good, and I attribute that all to the parallel flow condensor.

The electric cooling fan, and sticking with r12, are the only two things I would have done differently if I had to do it all over again.

GregS
'84 300D, 174k
'90 300CE, 164k