Maybe some of the experts can chime in here, but that all sounds normal to me. After all, it is a converted R134A system (87 was R12) and it will never be as cold as it was originally.

I have the same situation with my 380SE which was converted professionally about 9 years ago. On Saturday it was just under 90 degrees here, and the converted system was just what I would call "adequate." This was during stop-and-go low-speed driving around town. Although two of us in the front were fine, a back seat passenger did complain that it wasn't cool enough for her. The system is charged properly with almost identical cycling pressures to what you described, and I'm guessing vent temp was around 50. When I drive the car to/from work, the vent temp is somewhere closer to mid/low 40s and it feels great, but I average 45 to 55 mph.

My 300E, on the other hand, has original R12 and it will freeze you if you let it.

I'm going to tint the windows in the 380SE to see if the cooling is improved. If not, I will consider converting it back to R12...

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08 W251 R350
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91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280