Fantastic plane for sure, but I wish I knew which gauge is causing such a dramatic FPS hit on FSX...
I don't know how much you know about working with the panel config file, so I am going to go at this like you know nothing about it.....not meaning to be insulting, just helpful.
Here's a look at a standard line from a panel config.
gauge.1=RPM_1, 120,150,80
The first part "gauge.1=" is the code for calling a gauge and the sequence it is in in the panel config. The next part "RPM-1" is the name of the gauge being placed. Parts two and three "120,150" give the coordinates for the top left corner of the gauge in an X (sideways) and Y (up and down) placement from the top left corner of the panel. The last part "80" is the size of the gauge in pixels. SOmetimes you will see 2 number for size "80,70"....this will be for sizing none round gauges and tell how wide and tall the gauge is in pixels.
Make a back up copy of your panel config file at this point...because this is were you start messing with it.
To isolate the gauges one at a time to find out which gauge (or gauges) are causing your frame rate drops, you will need to change the line just a tab bit to look like this.
//gauge.1=RPM_1, 120,150,80
Placing the double slash // in front of the gauge line prevents the sim from reading that line. Start at the top and work your way down through the gauges, one at a time, until you find the one that causes your frame rate drop. If nulling out gauge.1 does not improve your frame rates, go back in to your pane config and remove the // and put them at the beginning of gauge2. Test again. It can take quite a bit of time and testing to find which gauge(s) are causing the frame rate drop. Be patient, don't fear messing wth the panel config (remember, you did make a back up copy of it..so if you trash the file so badly that it no longer works, you have your back up to fix things).
Hope this helps you find the problems with the frame rate drop. Again, I am not trying to belittle or insult you...but to help you improve your sim performance and enjoyment.
OBIO