The full span flap vane gave more lift BUT it needed so much back stick to trim that you could under some circumstances stall the tailplane.
With the T45 when more lift was needed to meet the required carrier approach speed the full span vane was restored and the rear fuselage had strakes added to stop the tailpne stalling. You can just see one in this pic (above the airbrake and behind the Y and in front of the tailplane) the shadow rather confuses things.
