As I said before, you have something else wrong. The A/C system in these Gen II cars is quite strong. Even with R134a, it should cool just fine and pull down the cabin quickly. You have an enormous evaporator, a large dual-circuit condenser, a large pusher fan (dual fans on 1988+ cars), a modern Denso compressor, and a good control system. Even at 100+ outside, you should be in the upper 40s, lower 50s running on 134a.

You likely have a bad monovalve or a bad pushbutton unit that is failing to turn off the monovalve. The entire point of the monovalve is to stop coolant flow through the heater core, in cooling mode it should be energized. If not, your heater core and evaporator are duking it out (and the evaporator will lose every time).

Converting to R12 may get you some slight performance improvement, but the difference isn't profound compared to a properly converted and charged 134a system. Even without window tinting, my 350SD has as good if not better A/C as most modern cars.

Also skip the "alternative" refrigerants. They *SUCK* at idle. Having been through them all (Propane, butane, Enviro-safe, R152a, Freeze-12), I can assure you that R134a has the best performance of any gas other than R12 you can put in the system.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]