I think you'll see a bigger difference between planes that had good diving capabilities and those that don't. The Mach effect, and forgive me if I get this wrong, has a lot to do with the drag of the airframe... so a clean aircraft like the Mustang will accelerate to its dive speed more quickly than something that isn't as aerodynamically clean. The J2M3 was fairly good at aerodynamics (if I remember right) so it may not give you a good example. Mustang compared to a A6M5 Zero might be a different story.