If you had used a little bit of nose down trim and kept the throttle full bore and WEP on, you would have hit the 500mph mark. Whenever I get into a diving chase against a P-51, that's what I do and I can usually catch them in a dive. The fastest I've been able to get the P-38 in a dive was around 525mph. I still had control because I was 8k when I finally caught up to the P-51.
It is also harder to enter compressability in late model P-38s (late series J models and the L model) because of the dive flaps and control is easier in high speed dives because of the hydrolic boosted flight controls. And also that the P-38 will only enter compressability at altitudes above 20,000ft where the mach threshold is lower because of the thinner air at those altitudes.
Here's a snippet that explains it. It's taken from the P-38 Online website.
Compressibility occurs what the P-38 entered dives stated above 20,000 ft. The airflow would be "splashed" over the leading edge of the wing instead of the usual smooth airflow. The splashed air would approach the sound barrier (not the aircraft itself, but rather the speed of the air flowing over the wing), thus causing a shockwave effect on the trailing edge. This would render the controls inoperable, leaving the pilot without any control of the aircraft. Two possibilities would then ensue. Either the aircraft would slow as it descended into denser air closer to the ground and the pilot would regain control and pull out of the dive. Or in some cases, the P-38 would simply disintegrate. Many pilots would lose their lives when they inadvertently entered a steep dive, or when performing dive tests. More importantly, this problem would affect the performance of early operational versions of the P-38, and also caused many rumors that plagued the P-38 early in the war.
Tony Levier described compressibility as, "It resembled a giant phantom hand that seized the plane and sometimes shook it out of the pilot's control." George Gray wrote in a history of the NACA, "The behavior was new to pilots, terrifying, baffling. Several men, in putting this two-engine fighter through its diving maneuvers, underwent the experience: A sudden violent buffeting of the tail accompanied by a lunging and threshing about of the place, as though it were trying to free itself on invisible bonds, and then the maddening immobility of the controls, the refusal of the elevators to respond to the stick." Use of elevator trim would sometimes bring the P-38 out of a dive before destruction. Sometimes the P-38 would begin to tuck under and begin to come out of the dive upside-down. Levels of stress on the airframe were staggering, and the fact that the aircraft would come out of the dive at all was proof of its strength. Hal Hibbard described the compressibility problem as, "…the air tends to be 'splashed' by the leading edge of the wing more of less like the prow of a boat at high speed in the water. As one approached the compressibility range, the air is throw to violently up and down the leading edge that is does not have a chance to flow over the wing in the proper manner."
Ack-Ack
