Spits do not out zoom the 51. At least not when starting from decent speeds. I did a zoom test way WAY back (AHII beta), the flight model has been tweaked since then, but I don't think that the conclusion will be different:
Spits are mediocre zoomers. However, the advantage of the "best" zoomers is quite small and will often not be more than 100-300 feet, meaning less than 100 yards. Even a slight energy advantage to the spit will result in it zooming to the same alt. The major factor that makes the light planes seem to be good zoomers is the great stall stability that almost ALL AH planes posses.
Normally, the player have no fear of full-power, extreme attitude stalls. This means that the hovercrafts gain an advantage not by zooming higher, but by zooming longer. The P51 will pull away only a little and start falling back while the spit/zeke/F4u(!) is still pointing up, hanging in there deep in the stall. It is not going up, but from the POV of the falling P51 it looks like it is closing the distance (but it doesn't get any alt at all). The little extra zoom of the 51 (~100 yards or less) is not enough to get it out of gun range and the stably hovering plane below can hose it down during this phase.
I blame the FM easy stalls on this. In real life, no WWII plane (I think) will be stalled until it falls tail-forward with full power on. If it does, the consequences will be severe. It means that there will not be a long hover period at the top of the zoom and power needs to be reduced to maintain control. E-retention becomes more important than power-loading and deep stall stability. The higher zoomer also zooms longer or equal in time.