Regarding the gain of airspeed at altitude by overevving the engine and "boosting" the supercharger this way, the F-1/F-2 Kennblatt gives some hint. Blatt 6 says that by increasing RPM to 2800 over the normal maximum of 2600 yields 10 to 15 kph increase in speed at the rated altitude.
http://kurfurst.org/Performance_test...1F2_DB601N.PDF