As with today, during the WW 2 period different nations had different standards for construction of their military aircraft. For example, the US standards often resulted in a heavier airframe than most European countries used. An example of this is North American's XP-51F and G fighters that were built to British load factors rather than US ones. This resulted in an aircraft that was almost 2,000 pounds lighter than a standard P-51D! This resulted in a more maneuverable aircraft that also had a top speed somewhere between 40 and 50 mph faster than the D model. The US also called for larger cockpits to accomidate larger pilots easily where say, the Germans often had ones that were more akin to racing cars and tight fits. I suspect that if one played around with this idea one would find many of the US designs would have greatly improved using European standards (British and German) while the British and German designs would have turned into true dogs with US engineering factors applied. The Japanese would surely have been hard hit using anyone else's design criteria; their aircraft being far lighter than other nation's in terms of airframe and skin requirements.