There is no chance changing the refrigerant to 134a will make it cooler. The only way it would be cooler would be if you were real low on R12 and filled it with 134. The system full of 134 would cool better than a 25% R12 system (sometimes even more).
The early 300Es were the worse of all models to convert (by that I mean they worked the worst - not that they were the hardest to do. They were the first MB we quit converting. The first car in general were 911 Porsches.
The recirc door element is one the back side of the evap and requires at least pulling the dash. The recirc door is partially closed by turning the A/C on and if this doesn't happen the duct temp will be 10-15 degrees higher.
If you wish to evaluate your A/C get a digital thermometer and see what your duct temps are. They should be atleast 40 degrees off ambient at 50mph or above and in HIGH blow.
Bosch Master, ASE Master, L1
33 years MB technician