The real genius of the design, IMHO, was that it was as a SHORT RANGE air defense fighter. It was meant to defend the skies over Britain, period..recognizing that the Germans would have to fly there, and back. Had they attempted to increase range, with large fuel tanks, and more weight it would have completely changed the performance envelope of the plane.
If I remember correctly the USAAF used modified Spitfires for long range reconnaissance using drop tanks. In fact they were able to fly over Berlin from the UK.