First, nobody ever reported GSX Pro to be "fps heavy" or "poorly optimized". In fact, most users would like it to be less optimized that it already is ( it's optimized a lot, and it shows ), and would prefer to have larger loading ranges for objects but, because GSX IS HEAVILY OPTIMIZED, the loading ranges are not what most users would like, but the fps is obviously very good.
Of course, without knowing anything about your system, and with how many other add-ons you are using GSX, it's impossible to understand why, a program that everybody agrees to be very good optimized with a minimal fps loos, to YOU seems the opposite. So, you might start by saying exactly which airport you use, which airplane you use, and something about your system.
About the supposedly "numerous bugs and issues which do not appear to be resolved", it would have been best if you said WHICH "issues" you are referring to, so we can go, one by one, and see if they are really GSX bugs, or just a result of either not understanding how to use it, or not understanding how to handle the sim shortcoming, for example regarding everything that revolves around jetways, which is a sytem full of issue that some users mistakenly thing are "GSX bugs".
So, please, make a precise and accurate report of the issues you think are GSX bug, and we'll check them, one by one.