Hmm, we had luftwobbles explaining the superior quality of the LW stock as well as quantity. but, that's...them.
They did get really low in quantity in the end, and AFAIK as well in quality, or availability of the right stuff.
They also had problems with engine durability, i.e. power loss due to wear and tear which has been claimed to occure within very few hours. LW aces as Rall have mentioned this after testing captured allied aircraft. Well, maybe this had something to do with boosting...MW and such, or lack of some material for engine building, - many opinions on that.
But the fact remains...the Western Allies took over the very high alt performance as soon as the Spit IX entered the game in 1942, and held it for the next couple of years, beyond the line when it was clear that the Axis would loose.