As a chemist, I have always been reluctant to believe claims that a/c conversions from R12 to R134a actually work. Aside from the "O-ring" incompatibility, R134a gets hotter on compression and essentially as cold on expansion. The final temperature after expansion depends on the initial temperature after compression. A/c systems have a cooling radiator to reduce this compression induced heat called the condenser.
The picture below (posted earlier by a fellow member) shows that the R134a has more area to handle a hotter fluid. This is the 1st time I actually see a factory difference between the 2 systems. R12 was a far better cooler in traffic. R134a systems do ok when the car is running and the condenser is well ventilated. Its a shame the industry was forced to change to a less efficient refrigerant.
Follow Ups
Cooling efficiency in coverted a/c systems - JMartz08:31:53 09/12/12