The P38's low speed handling completely sucks since 2.07. Once flaps are extended 75% or more it wont even nose up. I've noticed with full throttle, auto pilot and full flaps, the P38 loses alt. It feels like I am flying with "stall limit enabled" but I checked and it is in fact off. I've had 109's with gondolas loaded out turn me, which did not happen before. The only thing that flaps seem to do for the 38 is add drag and no lift. The new stall FM seems good for most planes; for example before 2.07, planes such as P51s and P47s would use full flaps to help them turn, but in WW2 pilots wouldn't dare extend flaps to that degree while in a combat situation. But for planes with Fowler/ Butterfly flaps it does not seem realistic at all.
I do not fly the Ki84 often so I do not know how effective its flaps are now compared to before.