I actually like the P51D, it does take some patience though and it's hugely over tiered I reckon. If a P47 is 3.0 and a Kitty Hawk is 2.7 ...a P51 should be 3.3 in my opinion. It is fast and it does dive very well and it can use the M20 incendiary tracer ammo like the P47, it's certainly better than the Kitty Hawk to me but it's hard to really compare it to a BF 109 G2 Trop which is also 4.7 BR, or the F4's for that matter, which are lower.
It's actaully on the 'same' battle rating as the Corsair F4U-1C which is packing 4 cannons and 900 rounds of ammo and is an absolute beast of a plane.
I think the P51D is a really nice plane to fly, it's just lacking firepower really. If the P51D was fitted with the same cannons as the Corsair F4U-1C it would probably be my favourite plane in the game, sadly it's not. The Mustang MK1 A (The British premium plane) which does have 4 hispanos just doesn't fly like the P51D, I like it, but I prefer the P51D even without the cannons.
I agree with Matt regarding the P51D's engine sound though ...what is going on there! ...it sounds like an oversized fan turned upto high RPM with WEP on.