Just did a test that explains it gentlemen.
With no drop tank rack, the Dora does run 375 on the deck on WEP.
However, with the rack in tow, the top deck speed is reduced to 369. And owing to the shorter legs on the D-9 on internal with MA fuel burns, I and I imagine most others usually carry the drop.
The P-51 is modeled with the drop tank pylons whether you carry them or not, and always runs 368 on the deck on WEP.
Therefore, a diving pony can should be able to make up quite abit of distance on a diving Dora with its greater E retention, under these conditions, once both level out, and this is why it seems like forever chasing a Pony sometimes in a Dora. You have to run them out of WEP before you have a speed advantage.
I have also noticed that a P-51 seems to pick up speed quicker with the nose down. I never run into compressibility problems even in exended dives with a closed throttle in the D9. I have in the P-51, (and the P-47, for that matter) and developed the habit of dropping a notch of flaps as speed-brakes when nessecary.
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