De facto: the FMC predicted max altitude you refer to is dependent on the stuff you feed it.
Again, your wing does not care about the FMC.
If you let the FAA airplanes believe their 25% Cruise CG default in the FMC, you might be in for a surprise. The JAA 8% will safely underestimate the values.
Cosmo Cramer, you take the words out of my mouth.