I made a little test. I hit autospeed 160 and let the planes glide (engine out). When sink rate was constant i lowered the flaps one notch.
The result:
P51, F4U, P38, LA5 : sinkrate stays constant
NIK: ~ -100 ft/min more sinkrate
190, 109 : ~ -250 ft/min more
Typhoon : ~ -400 ft/min more
So, when the american and russian fighters have with "flaps one notch down" same sinkrate compared to "no flaps", doesn´t that mean that they all have flaps that can give them in this position a increase in max. lift without more drag?