EdwardS, YES. FSX takes into account weight of the aircraft. if you want to stay on one cruise FL and on fixed speed - then you'll have to decrease power with time, so you will burn less fuel. or you can climb slow to higher alt, and save even more fuel for cruise. or you can fly same power setting, but you'll fly faster and faster :)

BUT! all that relies to flight model, not Fuel Flow rate. If you have A\T - it'll adjust power and you'll get less FF, in 727,707s you gotta do it yourself. And FSX default fuel planner sucks for payware aircrafts anyway, it only gives good results with default, and close to default crafts. no matter how it calculates EFU - it good for default crafts, and they all way to powerfull and superfuel-efficient.

To calculate estimated fuel usage with respect to all that, you will need some tables\figures for specific aircraft.
the bad thing about it - it all comes to how precisely flight model is :)