This may be true but out of all the WWII books I've read my understanding is that either the P38 or P51 approached the sound barrier it started to shake violently but did not break the sound barrier. Many pilots reported this phenomenon. Remember in the movie "The Right Stuff" when Chuck Yeager experienced the same problem of turbulence with his aircraft but once he went through it the plane settled down. This phenomenon was due to air compression against the plane the closer it came to the speed of sound. Once through it the air compression was behind the plane.
Truly, the violence in making the transition. Now I wonder about what I read, if it was mere folklore, or what. Can't see the Mustang going there, far too light and airy. That '38 though....a pair of I-beams with cross-braces...heh...that thing might have done it despite the straight wings. That Ugly Bastard.