It doesn't matter WHAT adds the weight. All that matters is that when players are testing the aircraft in question to get the weight of the aircraft in question as close to the listed weight on the chart as possible. The best way to do that is to load it up with the different options (fuel %, ammo packages, guns, ord, etc), record each weight, then up the plane as accurate and as close as can be to the weight on the charts and THEN compare. If in the testing some goofy numbers arise for whatever reason be sure and pass the info along to HTC using the bug forums.
When the Mossi FB Mk IV was remodeled there was a lot of debate on the speed and climb of the Mossi FB Mk IV. Which chart did the RAF use, which weight did JTCuse, what external apparatus was on the plane when it was tested (DT's? Bombs?), and at what altitude was it tested at??? I think I saw about 8 different actual RAF charts presented in one of the threads. Ultimately, HTC adjusted the Mossi to a faster climb rate based on that thread (or so it appeared anyways).
FWIW, in the case of the Mossi FB Mk IV the weight is not as much of an issue as is the drag. The Mossi can fly faster with 2 bombs only in the bomb bay vs 2 bombs only on the wings. Likewise, the Mossi gains about 2 MPH once the bomb bay ord is gone. The Mossi gains quite a bit more once the wing bombs are gone. The rockets rails are the worst source of drag, but the 8 rockets offer 1495 lbs of damage, so there is a trade off.