It should be noted that actual testing of the P-51 vs F4U-1 revealed that the F4U had a smaller turn radius with and without flaps. However, the P-38G was just about dead even with the F4U, again with and without flaps.
That is not the case in AH2, where the F4U-1 has a significant advantage. So, you can find some discrepancies between real world data and modeling in AH2, but these are the exceptions, not the rule. Tailoring flap performance for individual flight models cannot be anything but difficult.
I do know that the flap issues are something Pyro wants to address. I imagine that this will not be a simple thing to accomplish. Previously, it seems that changes to the drag model helped some aircraft and hurt others. There probably isn't a simple solution, so I think that we will have to soldier on with what we have for the time being, adjusting tactics for the individual characteristics of each type. In the mean time, I'm confident that Pyro will get his arms around it.
My regards,
Widewing