much like the Ta-152, Me-163, tiger II, wirblewind, ostwind, F4U-1C, Ar234, so if the P-63 ever gets added it wont be the only one that did not have a big impact during the war, also no one has mentioned that while not officially, it is thought that the soviets did use the P-63 against Germany, but on paper they were marked down as P-39 sorties due to the lend lease agreement, nothing that can ever be proved but something to think about none the less
edit, and if you really want to talk impact on the war, one could say that the Tiger 2 had a negative one, as they could have used the time and material to make more of the good tanks they already had instead of that massive POS that broke down if you glanced at it the wrong way