Maybe the reason the Luftwaffe fighters did not use drop tanks, was a case of geography. When RAF or USAAF fighters escorted bombers deep in to Germany, they needed extended range. But on the other hand, German fighters could be based in Belgium, France, etc, close to the channel and therefore reducing their flight duration for a raid, as even for targets in the North of England, distance would not be so great as flying to Berlin, for example, for our boys.
Not sure whether this is the correct assumption but it is feasable.
Richard