I remember that someone from the PMDG team(Ryan?) mentioned they'd succesfully used the NGX with TOPCAT. What are the recommended weights, and is the TOPCAT c/g valid?

I have only used TOPCAT to get data. I have not used it, nor will I , to load pax, cargo or fuel, nor have I used it to generate reports. This is what I do,Set up the plane in TOPCAT, then when you go to a flight, under W&B tab, press MAN ZERO FUEL WEIGHT, set up the dep and arr airfield. Go to teh take-off tab, on the left and for take off weight, press "GET", then run the compute to get the data. Same goes for landing. Make sure the weather is correct as well.