Probably replacing the drier and flushing the system wouldn't hurt things, but if it's working I wouldn't open it up to do either. I'm no AC expert, but I believe that usually the AC is spec'd to cool down to a delta of around 20 degrees, so if it's 71 deg or greater, then 51 at your vents sounds good. Maybe you can clean the fins on the condenser to make sure you get good air flow (prob doesn't matter sitting still at idle though). Once you open the sytem and flush it and replace the drier, then you have to evacuate it with a vacuum pump for several hours to remove moisture. Seems like a lot of work that may not improve things much. I've also heard that early R-134 systems just didn't cool very well compared to R-12.