the first thing to do with the carbs is to figure out whether you have Zeniths or Webers. Next, is to find out what settings you have. This will include the sizes for air correction jets, emulsion tube, main and idle jets. this will require mild disassembly to read the numbers on the parts.
There is a blow up weber diagram here:
http://www.pelicanparts.com/911/911_Parts/911_1969.htm
The carbs are easy to disassemble. Just do one side at a time so that if you make a mistake you have another carb to look at for reference.
Since altitude has less oxygen than FL Sea level, I suspect your car is now running lean. It is getting more air than fuel. You will probably notice that they stumble and possible pop through the top of the carbs. you can richen up the mixture by opening the idle bleed screws. If it still runs lean higher than about 3.5K rpm then it will need larger main jets.
Once we know your current sizes, we can make recommendations for changing them. You may be able to call the PO or his/her mechanic and get the jet settings.
Jamie