1987 420 sel a/c

I took my car to my MB certified indy shop today because my A/C is cold on low and cool on high,I want ice cold! They evacuated it and recharged it (with dye).It had no leaks and the machine indicated it was operating as it was intended.My indy told me that because the evaporator is copper instead of aluminum and we are in such extreme heat and humidity( South Florida) that this was the best I can get unless they exchange my evaporator for an aluminum one, which they did not recomend. Anyone hear of this before?