I also believe that HiTech has his personal airplane also in the game so he can use it to compare it's performance with real life data and for the odd evil con mission.
But as someone linked to wwiiaircraftperfomance.com, you can find some of the official data that was used and in books and government sources as well. Also, people have to remember that due to the complexity of creating a perfectly accurate flight model, it would be beyond our feeble desktop PCs to be able to process it so allowances have to be made.
For a PC flight sim, Aces High has a pretty damn good flight model and physics and there is a reason that after 10+ years, the game is still around and doing well.
ack-ack