Except, at speed, the Spitfire (and 109 for that matter) had trouble with the ducts stalling and losing the effect. That's why, when the Mustang came over and proved to be so fast there was tremendous interest in the laminar wing, as it seemed the speed couldn't be due to the problematic duct effect. Real world experience, however, proved the difficulty of maintaining laminar flow in practice (Langley also doubted its field practicality). According to Lee Atwood, only two WW2 aircraft really made the Meredith effect pay off, and that was the Mustang and Mosquito and this was the biggest secret to their speeds.