You cannot really compare the flying qualities from one flight sim product to the next, because each is only an interpretation of what a particular developer may know, may hear, etc. In real life, the flying characteristics of the P-51B are generally the same as the D, except that there is much more yaw instability. One would think that with the larger fuselage area that the aircraft would be more stable, but this is simply not true. According to current pilots who have had the fortunate opportunity to fly both the B and D models, the D is a better flying aircraft, but the B is more fun to fly, since it is a bit more demanding. The common D model has the extended tail fin, which added a lot of directional stability, and the D wing has a bit more wing area than the B. The B is also more of a hot rod than the D. With the D model, there is quite a lot of drag that develops off of the teardrop canopy, where the air behind the canopy gets turbulent - I think that is the right way to put it. The B will generally fly 15-20 mph faster than the D, no matter if it has the birdcage or Malcolm hood canopy.
With my settings always having been at full realism, I have never found the plane all that difficult to take off and fly, though the experience I believe is closer to real life than what I have seen done yet so far. With practice, and reasonable power settings and correct takeoff trim, it shouldn't really be difficult at all to keep the aircraft straight down the runway. There are four forces that you experience whenever you power up and head down the runway in a high-power prop fighter - they are spiraling slipstream, reactive torque, p-factor, and gyro precession, and I go into detail about these forces in the manual that I wrote up for the release. You will feel the effects of all four of these throughout the takeoff run, from your initial power increase, to when you raise the tail off the ground, to when you fly off, and even when you are doing any maneuvers through the sky.
The aircraft can power up to 67-in of manifold pressure, without a stop on the throttle. You can actually takeoff in a Mustang at as low as 40-in MP, with an average weight. Typically, pilots today takeoff at 45-55-in MP, and no more, sometimes 40-in when taking off in formation. I find it best to hold the brakes on, at the end of the runway, power up to 30-in MP, then release the brakes and smoothly throttle up to 50-in, giving the nose a little forward pressure once I reach about 70-75 mph. The big gyro created by the prop disc will really start to become noticed when you raise the tail, because you will be changing the angle of attack that the prop disc has towards the oncoming air. With more pressure at the top of the prop disc, which is rotating to the right, it will want to send the aircraft to the left - this is the one major point to really look out for on the takeoff roll. When you lift off, you are going to need at least some right stick as well, since the aircraft will want to momentarily start rotating to the left once it breaks ground.
Also, I really recommend not flying with the fuselage tank full, or having any more than maybe 25-gallons in the tank as a reserve, as it will really give you a bad experience flying the aircraft.