It is significant enough that they mentioned it in the text. If you think those flaps and the disrupted airflow they create won't produce significant drag then you're deluding yourself. Not my problem though...
"DIVE RECOVERY FLAPS.—P-38L and Later P38J
airplanes are provided with dive recovery flaps to improve
the dive recovery characteristics of the airplane.
As described above, the airplane without these flaps becomes
very nose heavy and starts to buffet above placard
dive speeds. This condition is caused by a high speed
stall and a consequent decrease in lift in the wing producing
the nose heavy condition. The dive recovery flaps
which are installed under the wings between the booms
and the ailerons, restore the lift to this portion of the
wing and thus cause the uncontrollable nose heaviness
to occur at a higher speed. The flaps also add some drag
to the airplane, which in conjunction with the higher
allowable dive speed, permits safe dives at a much steeper
diving angle. The dive recovery flaps should be extended
before starting the dive or immediately after the dive
has started before a buffeting speed has been reached. If
the airplane is buffeting before the dive recovery flaps
are extended, the buffeting will momentarily increase
and then diminish. With these flaps extended, the nose
heaviness is definitely reduced but the diving speed
should never be allowed to exceed the placard by more
than 15 or 20 mph. With the dive recovery flaps extended
before entering the dive, angles of dive up to 45° may
be safely accomplished. Without dive recovery flaps extended,
the maximum angle for extended dives is 15°.
Diving characteristics are better with power off than
with power on."