It would have been a better idea if they did it before invading Poland, but people may have asked questions about reduced living standards.In my opinion, any "better" plan of RLM and LW makes sense only if the German economy goes on a total war footing immediately, in Dec 1940.
Otherwise, the Luftwaffe machine will run out of steam in winter 1941/42 as in real life, even with better equipment or smarter tactic.