HI, I, too, am dissappointed with the performance of my ACC (1986 420 SEL) after the conversion. I understand the condenser performance is part of the problem; I looked, and don't think I can get a larger condenser in there. If there is a replacement/upgrade condenser that will make the system work, I iwll buy it (I have to be convinced it will actually work, and not just help someone send their kid to college). What about adding condenser by plumbing in some finned condenser tubing like the Spine-Fin stuff GE uses in their A/C units? It would fit nicely behind the grill in the plastic bumper cover, at least 1-2 passes about 3 feet each.
Ultimately, I am probably going to convert mine back to R-12; I hate to make excuses to passengers for the poor performance; it's embarrasing.