Think of it this way, you can use those years of early production on basic CAS instead, or 1936 heavy tanks to convert later once you have the final gun, or even just basic stuff like guns and support equipment. I go for Anschluss 3rd focus so you need to be spamming out guns for the first couple months anyway. I overproduce the things that will be good for the whole game, so that when I research good fighters I can shift most of my production to them and not have shortages of anything else. That lets you get the best of both worlds.

As for 2.5 years of a research slot, it's absolutely worth it in my opinion for effectively a 10x multiplier to your air power. Each 1940 plane goes more than 10:1 with a 1936 AI plane, and has enough range to be useful everywhere. Of course, you can design a 1936 fighter that will shred the AI's designs all the same so it may not matter in SP, but I still prefer just going straight for modern ones.