Wozza, I think the idea of the blitzkrieg economy is just an academic attempt to explain away the slap-dash way Hitler and the Nazis ran the German war effort. As if bad planning and up-down procurement were an actual plan, rather than the result of government by dickheads. With quality central direction, resource allocation, prioritisation and addressing bottlenecks and other problems the German economy c/should have produced twice of what Britain did after 1940, the industrial base was there.
But there is no blitzkreig economy - there is no "protection" of living standards and there is no evidence of a conscious decision to favour breadth over depth. It is more likely that Hitler was considering both short wars and long wars, in fact we know that he was from his assorted statement. The two goals are not irreconcilable.
The problem with this "bad planning" line is that it is just one more variation of "blame Hitler;" the idea that German was tactically brilliant but strategically awful - only in this case that Germany is a misdirected economic powerhouse rather than military juggenaut.
This fails on two fundamental points - in comparative terms German planning is not that bad; and it is not a powerhouse anyway, there are deept problems that better planning would be unable to solve.
That is why going back to the original question, I do not think it possible, given a realsitic POD, to deliver more than marginal improvements to German production.
Planning:
All the powers suffered from poor co-ordination, vested interests and poor arbitrary decisions. Did Germany really suffer from more? Possibly at a certain level, but having a joint services committee simply shifts as many problems as it solves, the idea that the failure to co-ordinate production earlier lost Germany the war is laughable.
The more important point is the limitations in the German economy (and here I disagree with esl fundamentally.) The German economy was simply not that advanced, certainly not compared to the US economy, and in many ways it was behind the British economy.
Yes Germany had some first rate, world-beating firms at the cutting edge of technology: behind that though, the picture is not that good.
First of all look at agriculture. During WW2 Britain had 1 million workers in agriculture, Germany 10 million. Even once you strip out differences of population and levels of food self-sufficiency you can see that German agriculture was massively inefficient. If it had been as efficient as British I imagine the Germans would have had 6 million more workers or soldiers - now that would be war-winning.
Other than the major heavy industrial concerns a lot of German industry was traditional craft firms, leaving overall manufacturing productivity way behind America, more on a level with supposedly declining Britain. On service industry German productivity was way behind Britain.
This backwardness was reflected in a number of ways - one indicator was car ownership, in the early 30s German car ownership was lower than in Ireland.
These are massive, entrenched problems, they involve millions of workers and thousands of firms/businesses. Raising the productivity of so many diverse organisations is a massive task.
First of all consider agriculture - you would need mass mechanisation, which would require huge industrial resources, in a country with a motor vehicle shortage....
The Four Year Plans were introduced to modernise industry, and they did, but they ate into resources that could be used for weapons in the early years of war. The Plans also involved developing new plant, and the expansion lowered overall industrial productivity (because it involved, for example, opening up less accessible coal seams.
Once into the war dramatic improvements in efficiency were made, but they were in the other war economies too. But ultimarely these would always be limited by the industrial base you start from - hundreds of small, dispersed workshops cannot be converted into a Taylorian assembly line without a massive investment of resources. The most efficient solution might simply cost too much in the short-term: certainly for the entire economy.
And this is an important point: effiency is needed in non-military sectors, it is all very well having tanks made in high productivity factories if the clothing factories still take up three times as many workers.
Germany starts the 30s economically backward in too many ways: there is simply too much to do, in too short a time to find a magic bullet, either in efficiency improvements or expanding capacity.