Judging the efficiency of the air-con by the cab temperature is a tad mis-leading. Many things can influence the temperature that the cab gets to. As already mentioned, removing the bulkhead makes a huge difference, as does adding glass, insulation etc. The general test for whether the aircon is working or not is to test the air temperature coming from the dash vents when the aircon is on lowest temperature setting, medium fan speed & on recirculation. Give it about 10 mins & measure the air temp coming from the vents, it should be around 6degC give or take. If this is the case, then the aircon is working & the cab temp is down to external environmental factors.