"I think the closest a prop driven aeroplane came to the speed of sound was a Spitfire at Farnborough which reached I think .91M, overspeeded and the prop came off."
Another contender must be Flight Lieutenant Ted Powles flying Spitfire
PR.19 PS852 of 81 Squadron from RAF Kai Tak on 5th February 1952.
The full account of his epic flight can be found in "The Spitfire Story", Alfred Price ISBN 1-85409-514-5.
51,500 feet true, Mach 0.94, 690 mph.
Chiarain.