Actual Flight test results with Spitfire II with 100 octane fuel:
547 km/h at 5000 m altitude (339 mph at 16 400 feet)
460 kmh/ at 0 m altitude (285 mph at 0 feet)
So we're gonna believe a German flight test cause 'cause Kurfurst likes it...that's funny. What's even funnier is it describes the Spitfire II as using a Merlin III