I agree. People making aircraft for FSX/FS11 should use the real values for the wingspan settings. Mixing and using FS9 aircraft mostly work anyway, but can be a real issue in single cases.
Some weeks ago I would have said that with the arrival of FS11 this will be a thing of the past as would have been FS9. Now that we ended up with two last versions of the Flight Simulator I guess this will never get really solved.
