What are you talking about? Spits don't kill flaks, flaks kill spits.
Spitfire mk XVI was designed for air to ground operations. It's engine is tuned for best performance at low altitudes and it's wings are also clipped for best performance at low altitudes. In addition unlike other Spitfire models they gave it three hardpoints.
The reason why you say that flaks kills spits is because in AH most people who fly Spits are noobs and don't know that they should stay away from ack, or when attacking it they don't know how to properly do it without getting killed. Given that both people have equal skill level, the one that is flying a Spit will kill any wirble or ack gun. You don't even need bombs to do it, with those 2x20mm canons you can easily take out the turret on the wirble.