Why not the P51B ?
What do need from the P51D you cannot be without ?
I normally fly outnumbered in Eny 20-30 plane-types trying to survive by flying smart, with squaddies to increase survivability.
A P51B is faster than any of them, can out-turn then all (with correct speed), and keep energy in turns like no German plane can, it carries more ordnance and can stay up 4ever.
The 4*HMG, is better than a standard 109g2, and equivalent to a 109G6, and not far away from the 2*20mm gun FW190A5. I would not try to HO a 190a8 in it though).
The P51D should be higher ENY'd than the monster 3*20mm incredibly rare LA-7, but I rather see that perked, and tag it ENY5-10 with one 20mm.
I don't like the 51B.
51D the most overrated ENY in the game. It's fast. That's about it.