A similar increase of RPM was permitted for the Merlin III (
http://forum.1cpublishing.eu/attachm...4&d=1334674718,
http://forum.1cpublishing.eu/attachm...5&d=1334674727), not sure about the date though. This didn't increase the maximum power but increased power of the 30 minute climb-power.
I think the benefit of increased rpm in the DB601 could be even greater if the propeller wouldn't be operated above the best efficiency at this rpm.
Would be interesting to compare Merlin III vs. DB601A(a), Merlin XII vs. DB601N (old supercharger/new supercharger, Bf 109 E/F) and Merlin 45 vs. DB601E (Bf 109 F-4).