The XP-39 is easy - take a look at pg. 84 to 85 of 'Vee's for victory' book, and especially at the graph at pg. 85 that gives 340 mph at 20000 ft for the 'original airplane [XP-39]'. No wonder, the XP-39 was draggy affair, with Cd0 = 0.0329.
The XP-50 - I have no firm data that it never did 425 mph, however there is also no firm data that it did. As a reality check, we can look at the P-38J, that bested 420 mph mark with 2 x 1600 HP (ie. 2 x 400 HP more than the XP-50 was supposed to have), while without the drag of the R-1820s to act as airbrakes.
added: the figures for the XF5F are even more
funny, to be polite - it is/was trumpeted as capable of doing 383 mph at S/L (= faster than
a P-47D with water injection and 150 grade fuel - 2800 HP - by about 40 mph), while the USN data sheet gives just above 310 mph at S/L.