Wing length is completely irrelevant to turning without knowing a whole lot of other numbers as well.
Wing loading favors the A-20G (when not laden with bombs and fuel, which Cobia's never is) over the P-51D's. What makes you think the P-51D should turn a tighter radius? The airfoil difference would need to be huge to make up the wingloading difference. Powerloading at WWII levels of powerloading don't make much difference.
For what it is worth, in the Mossie VI, which has very close turn performance to the P-51D in AH, I never try turn with A-20Gs and they are helpless targets. Even Cobia I'd think, though I haven't fought him.
Just don't take liberties.