The simple truth is that one a basic level of competence is reached, the plane is far more important than the pilot.
It is possible to quibble over the details all you want, but the simple basic truth remains unchanged.
Is it impossible to kill a Spixteen in a P-40B? No, absolutely not. As long as the Spixteen pilot is absolutely brainless, it is certainly possible. If the Spixteen pilot has even an inkling of a clue, he will know that it almost doesn't matter what he does, he will be able to kill the P-40 - and it doesn't matter who is in the P-40. You could give a good teacher (Widewing, Ghosth.. don't really know any other trainers... but any patient person with a clue) a couple days with a rank noob, teach him how to seize every advantage (including the plane - i.e. an La-7 or Spixteen), put the guy in the DA with Leviathn and the rank noob will win 9 out of 10, as long as he keeps his wits about him. Levi would be bored out of his whoopee skull, but he would die, every time. About the best he could hope for would be a MAD HO joust, but even then the Spixteen packs 10x the firepower.. so I wouldn't even recommend that.
Why do you think some planes just don't see ANY use in the MAs? A good pilot can take an 'average' plane and do well in it. A great pilot can take a crappy plane with one good attribute and have moderate success in it. Certain planes (P-40B, Spit I, 109E, that crowd) don't even have a single good attribute to exploit against a mostly incompetent opponent in a 1945 plane, nor the firepower to take advantage of the countless mistakes the initial opponent will make.