I am not too far from Arlington Tx. I live in West Monroe,LA about a 5 hour drive and our weather is similar to yours. My car is a 86 300sdl with a retrofit to 134A. I have no problems with keeping the car cool unless it sits all day in the 95 degree heat. Then it takes about 3 miles on I-20 to cool it down, but so does our Honda Odyssey. My daughters 2000 Toyota Camry blows ice cubes sitting in the parking lot just after I start the car!!! There is something about Toyota Air conditioning.
I did notice that the last Tech that checked my A/C had my system a little overcharged in the Benz. I released some and it cools a lot better. I guess that is because the 134A expands more?
Anyway, my Dad has a 380SE with a 134A retrofit and he tinted his windows and that helped a lot. His car being an 85 does not have a recirc button like my 86 does. One thing I checked and replaced were my vacuum actuators for the main outside air flap. They were not closing all the way when the recirc button was pressed in.
I am tempted to go to the new type condenser when my system fails, but until then I am leaving well enough alone.
Oh, another thing that helped my A/C was blowing the crud out of my evaporator drain lines.
John