As I understand it, the Spitfire Mk.XIV employed a "slipper" tank, beneath the fuselage, which did not use a rack. The tank was fitted directly to mounting Points on the underside of the fuselage. Once jettisoned, the aircraft was "clean", with no rack to cause drag.
Note also that those Mk.XIV's used by ADGB in the anti "Diver" role, against the V1, were also lightened by removal of armour plate in some instances, and had the rear-view mirror removed from the top of the windscreen to give a slight speed increase (from memory, around 4 mph).