Some people probably showered their drink thru their nose, reading that. The 51D's definitely near the top of the short list of best fighters in the game.
How so Moot? As a bnz'er, hunter plane, its okay I guess. But many planes fill that niche well. As a 1v1 fighter? It turns nearly as badly as the 190s, without the climb, accell, roll rate, or firepower. It climbs and accelerates only a little better than the Corsair, while being vastly inferior in turn. P-51 zooms well, but no better than the Hog. A freakin' JUG can beat it in angles fighting right now. The P-51's gun package is not particularly deadly. It is not even the best runner.
The fact that is flown by some of the best pilots in the game and they fly it extremely does not mean the plane itself is that great, as modeled in AHII.
Short list of best fighters? Here is my "short list" of planes that will generally kill/drive off the Pony in a Co-E fight between equal opponents:
Corsair
Spit
109
Ki-84
P-38
N1K
And thats just off the top of my head, without going into sub-models.