I would think a stock system should do a little better than that... You're not going to get too much more. Your evaporator might be plugged up with debris so you're not getting good air flow through the fins of the evaporator. Did you have a good shop make the change over from the R12 to the R134? If this wasn't done correctly you might be losing some temp there.
What kind of cooling were you expecting? I'm sure you're aware with after market modifications that are now available you can achieve much lower temperatures if you're willing to either do some work and or spend some money.
You can do a search here to find tons of information on cooling the Porsche... Here's an A/C Tech Article on an "84":
http://www.pelicanparts.com/techarticles/911_ac_1984/911_ac_1984.htm