The notable reforms that did happen were organizational, most importantly the advent of the Zentrale Planung, a council of the war economy’s leading figures to administer raw materials and thus something of a commanding organ. Tooze gives the Speer ministry’s intervention in the Mark XXI U-Boat program in 1943-5 as a case of substantive changes to production in the name of “rationalization” with all of its tenets: minute attention to detail, American-style economies of scale and outsiders overturning the will of conservative industrialists. Construction was distributed into modules built inland and assembled in the docks. It failed: inland manufacturers were inexperienced, the administrative apparatus not ready and the design itself incomplete. The promise of 30 Mark XXIs by summer 1944 was never fulfilled. Speer’s rhetoric of rationalization deserves skepticism.
Within the Luftwaffe’s large share of the economy, not under Speer but under Milch, there was indeed efficiency-driven growth, with production expanding with negligible extra labor and aluminum. This was in part because through 1941 there had been organizational advancement and a tightening of control over Junkers, BMW and Heinkel, but mainly for the decision, in the wake of developmental disappointments, to ditch quality for quantity and focus on proven old designs, allowing economies of scale and the accumulation of experience.
Overall exponentail growth was related not to efficiency but to inputs and stability. In 1942 the inflationary threat was suppressed, a food crisis controlled and the wide-scale importation of millions of foreign workers began: by 1944 a third of armaments workers were foreign. Though it took time to reach such numbers and their integration into the economy was a lengthy process, it was still a large influx of labor. And more important was a heavy industrial boom, with steel production rising until the strategic bombing campaigns of March-August 1943. Coincidentally fast growth of arms output also stopped at this point. That Speer’s “miracle” was a matter of inputs, not efficiency is clear in how his Ministry’s prestige projects, such as locomotives and missiles, were generously provided with labor and raw materials, and even more so in how closely ammunition production followed its steel allocation.
The output of 1942-3 was by far not the result of a rationalization that could have been done already in 1940-1. Some things could have been done earlier, such as organizational advances and consolidation of control over the air industry, but their impact would be modest. Hitler could not have simply appointed Speer in 1940 and gotten +10% Industrial Capacity.