From the Pilot's Handbook of the Me262 written by 1st Lt F D Van Wart of the Air Materiel Command, Wright Field, Dayton for the USAAF pilots evaluating the aircraft.
"Speeds of 950 km/hr (590 mph) are reported to have been attained in a shallow dive 20 to 30 degrees from the horizontal. No vertical dives were made. At speeds of 950 to 1000 km/hr (590 to 620 mph) the airflow around the aircraft reached the speed of sound, and it is reported that the control surfaces no longer effect the direction of flight. the results vary with different airplanes; some wing over and dive while some dive gradually. It is also reported that once the speed of sound is exceeded, this condition disappears and normal control is restored".
This was published on 15 Jul 46!