Some things HTC has chosen to keep in house.
Partially to keep every plane to the same level of realism in modeling.
Possibly to prevent people from being able to change the data. (800 mph p51 that turned like A6m)
Skins yes, but there is much more to what goes into a plane than how it looks.
One of the truly hardest parts is simply finding accurate data on flight characteristics.
Speed, climb, turn rate, that is proven data for the right model, & the right conditions, that isn't slanted or subject to personal impression or highly subjective pilots interpretation.
Example, there is 1 A6m zero in the world that fly's, its at Chino, but even it is running on much different fuel than it used in the Pacific. And I'm pretty sure it has got a different engine in it.
Which makes it basically useless for modeling plane data, as you've already got 2 points of inaccuracy.