Hmmm, I thought the 'Speer Boost' was more of an accounting slight-of-hand made into historical myth. Earlier historians point to him 'increasing' fighter production, but recent historians recognize factories had switched from panzer engines to fighter parts months earlier, in response to Cologne and Hamburg burning down... In any case, though, the Germans did bungle the wartime economic mobilization pre-Speer.Nazi Germany was very corrupted and mismanaged state. No wonder Speer managed to boost significantly German production in the end of the war.
More on-topic, Lend-Lease probably helped tie up Luftwaffe resources in the west, even pre-Stalingrad. Losing pilots, planes, and other assorted resources over England wasn't fun for the Nazis, though really, I'm not sure how much resources are freed up if Britain loses Lend-Lease but refuses to peace-out...
What really *is* the question? Are we asking if the war can be won without *any* American involvement? Because if the UK only works on its own resources, then they might fold to U-boats, and we get all-in on the Eastern Front, maybe. Are we suspending Allied aid to the USSR? Because without US-USSR co-op, then Bagration is crazy-Nintendo-hard, Kursk is tricky, though Stalingrad/Uranus probably muddles through. Expect 2+ pages on whether counter-factual Stalingrad is a grand Soviet victory like it is in earth-history, whether alternate-Stalingrad is badly prolonged but still a USSR win, one AAR where it devolves into a 2 year stalemate along the whole Volva, a couple voices claiming possible German victory, and one claim of alien invasion.
Jonathan Fisher