Porta's link is a great site. I know the author (he posts here sometimes) and he has really done some great research on the Spitfire and other RAF aircraft.
Naudet, the dead spot in the climb rate is due to the supercharger gearing. It doesn't go to high gear until a very high altitude. This hurts performance at medium altitudes, but performance at 25k and above is fabulous. It's one of the few planes that can do sustained hard maneuvering at those altitudes.