it's not how cold the air is in a 123/6, it's how much air there is and how much it's mixed with outside air that's the biggest problem.
our cars have a small blower, and a big car. the ac is matched to this small blower.
R12 is the best cooling you are going to get, so if you can, have your windows tinted, and cut down the solar gain your car is trying to cool. it'll help.