About the fastest piston aircraft. I've read that it was, in fact, a Spitfire.
Apparently, on the 5th February 1952, a Mk.19 of 81 Squadron, Hong Kong, flown by Fl.Lt. Ted Powles, went for a routine meteorological survey, but rather than stopping at 50,000 feet, went to a true 51,500. Cabin pressure fell, and, whilst diving down, entered an uncontrollable dive, before getting control back at 3,000 feet, before landing safely. During this dive he hit a speed of 690mph or mach .94, which makes him the fastest ever pistorn aircraft.
Of course, it wasn't planned, and wasn't in level flight, which I believe is the usual criteria for measuring speed records. Still, this is still a record, in it's own right!
(I was looking up something on the Spitfore on Wiki months ago, when I came across this)