I really don't think the US economy was as flexible as the Axis economies for the simple reason the US economy went to a total war footing immediately.
Worr, out
Hmm. I don't believe that German economy was that much more flexible. The German Industrial might did not get into full swing (wartime footing) until 42 or 43. The output of fighter aircraft, for example, was greater in '44 than the previous years, despite the full scale bombing effort by the Allies. The German economy was left at peacetime production longer for two reasons: 1) The war was assumed to be a short one, and 2) The populace needed to be appeased, in that, the regime in Germany needed to make it seem as if it was better at running this war (and the economy) than the Weimar government, and to combat any potential war worries.
In the prewar years, the bf-109 was awarded the lucrative Luftwaffe contract despite being beat by the Heinkel entry. This favoritism to Krupp, Messerschmidt, and so forth continued during the war years. The President/Chancellor might have been a tyrant, but he still needed the industrial czar's to fund and equip his war aims.
Ditto for Imperial Japan. I can make a similar assumption for Italy.
USSR is a big blank to me...
Point being, in terms of flexibility, the US was as flexible as anyone else. Maybe more so, as the US President did not have as much legal power as the German one... but that is speculative.
I have never claimed that the US would produce stuff out of thin air faster than it historically did, just because Australia or the West Coast was invaded. But I did postulate that stuff earmarked for the ETO might get diverted.