It will depend on the representativeness of the modeling.
If your fuel system model, your engine model, and your aero model are representative of the aircraft, whatever is the qualification of the simulator, you will have good results. However, it will never be as accurate as the aircraft because of the unprecision of the modelisation and the fact that we are not able to modelise the thermodynamic part of the engine. We are using tables to get that and it creates a lot of inconsistency.
For example, if you try to look at the engine consumption in cruise it will be pretty accurate, but during acceleration phases, it will not be accurate at all due to the inconsistencies.
A simulator qualified as FNPT II + MCC under EASA or Level 5 under FAA should have the same representativeness as a Full Flight Simulator Level D for all the models talked above. So it will give you a good idea, but will never be accurate. However, it is still the best you can have from a simulation.