I'll just throw in my two cents.
The P-51, when trimmed correctly and flown at the right altitudes is unstoppable. it will outrun, outdive and typically outzoom just about any other comparable blue plane, that isn't a jet or a late war super-prop.
However, it does have a problems, the laminar flow wing stalls at a higher speed than more conventional wings, and it struggles to build energy down low (mustang III and it's 150 octane fuel notwithstanding). This means that fighting down low and slow is no. Also at higher speeds, the 109's controls become heavier, probably leading to claims that the mustang was more maneuverable.
To make a long story short the mustang is an energy fighter, and probably one of the better ones. Close in dogfighting is better left to aircraft better designed for it, such as Spitfires.
I know nothing about flying in real life, but I do remember reading a post many years ago, on a WWII Fighters forum, by a US WWII fighter vet (probably long since dead, RIP) who said that planes were generally modeled on flight sims as being much harder to fly than was the case in reality. In particular, he asserted that it was much rarer to stall or spin in real life than in the games. Therefore, reducing difficulty settings in Il-2 may actually increase the realism.
Of course, how this impacts the relative ability of various planes is another matter entirely. Of this subject I can only say one thing with certainty-unanimity is unachievable!
When you fly for real, you can feel what the aircraft is doing, and so it is easier to tell if you're pushing it too hard and are on the verge of departure. Unless you have a force feedback stick, it's harder to tell in sim.