Definitions
OK so we need a definition - 'piston engined singles in production and service before the end of the War'.
What about someone 'normalising' the quoted speeds at varying altitudes to a common altitude?
Or consider the Blohn & Voss Bv 155B, also a star of "Aircraft of the Fighting Powers" Vol 7.
Although only 2 prototypes flew, they were (allegedly) capably of a max speed of 428mph at 54,000 feet, thanks to a 'supercharger system [which] resulted in a loss of power output of only 380 hp at 49,000 feet'.
That's 'Aircraft of the Fighting Powers' for you!