the P51 is great at retaining energy, not so good at gaining it back especially on the deck any weight shedding you can do greatly helps it.
Try running the P51D with 4 guns and 25% fuel with drop tanks. Ditch the drop tanks when getting into combat, run down the left wing tank first and you have the right wing tank to get you home, also this helps balance the torque from the engine at the top of loops and other low speed situations.