I have nothing to do at the moment so, my opinion is the Spitfire did much more effecting Axis defeat. I think the author of this thread has nothing to do either although I haven't spent enough time to see any 'Mustang won the war' threads. even with US versions of WWII history and watching documentaries, off hand I don't recall such a statement.
How much did it contribute to Winning the war...
I'd have to go with 0% simply because it didn't enter extensive serice till mid 1944 by which time the war was already won...
It contributed to the allied bomber campaign and campaign in the west and in some small way contributed to the shortening of the war...but i've always believed that the effect of both the bomber campaign and the impact of Allied Air superiority on the ground campaign have been greatly over estimated.