Ranger45,
If there is a difference it is very minor, from what I have heard from trainers. There are some planes that need to be remodeled; but that will be fixed when they get around to it. For the most part, the game is as accurate as they can make it.
I am sure they obtained information from the companies who made the planes, historical data, pilots, books, historians etc., etc.
I do not foresee HT giving out any information on how they design the flight characteristics in this game.