ok, did a couple of flights experimenting with my -6: no discernible difference in speed by fully retracting the flaps.
I already noticed that said flaps set themselves up in a higher position whilst in flight, pressure on bottom and suction on top.
Now pushing the release knob on the flap handle and pushing the handle fully down shows maybe 1/2 knot speed decrease
. The handle will then happily maintain that position.
Guess all ships being built with slight variations will show different results