I agree, the coolant has nothing to do with initial cooling via the AC.
When was the last time the AC was serviced? Have you had it checked for leaks? You can have a fluorescent dye introduced to the system that enables a tech to easily find a leak.
Is your engine belt tight? Any squealing recently?
Also vaccum the temperature sensor which is in the interior light/sunroof switch cluster up above. I did this on a 300SD and it made a big difference in cooling.
Also, there is a tube that connects to a small fan which pulls air through this sensor, it could be your fan is no longer working or the tube has rotted away.
The system also allows outside air into the cabin via a blending. Sometimes these "doors" or their actuators fail allowing too much air into the cabin.
Per the previous suggestion, have you tried using the recirculation only to see if the cooling improves?
Keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)