P 51D right now outdives my 109, unloaded or not.
Greater plane weight and better aerodynamics seem more important than engine power. Also, lighter plane can't pick up speed as fast as heavy plane in dives.
The usual scenario is:
1. get on P 51 six position, some d5, moderate closure
2. he rolls over and hits for the deck
3. he is away, no matter what I do to follow down
Some come back for HO, some dive home.