It would need to divert ~70 IPCs to break even, yes, which means that it’s kind of a wash as far as time is concerned. To really make it worth it, you’d likely need at least a couple more.
Will it defend Moscow?
Yes, even with a crippled Economy, Russia is still producing infantry, and putting fighters on top of all of that is devastating for German hopes of goose stepping around in front of the Kremlin. If the Germans divert forces to clean up all of that delicious, sweet NO money in the south of Russia and Middle East, then this investment of fighters likely holds Moscow for quite a long while. We’re talking about turn 7+ of the game at the moment, so it’s very, very hard to make big generalizations, but I’ll pull a number out of my ass and say Moscow would be saved at least until round 9, and in the situation you’ve described, of growing German economic power with Germany steadily increasing in strength, Germany usually isn’t interested in taking less than very good odds (90%+, or even higher) to smash Moscow since it has nearly everything to lose and not much to gain by rushing in, so you could hold it until round 10-12, maybe more, depending on the game. And that’s just with 10-11 American planes and maybe a few more from UK in the later turns.
Does it really matter?
Well that’s the important question, and it’s a much harder one. You’d be depending on the UK to be holding its own in the middle east, out of some minor ICs wherever the UK feels like putting them (I love Iraq, and in this game, since the US is taking care of air support for moscow, you could easily plop down a second). The American fighters can also fly back and forth between Moscow and a UK stronghold in front of the largest German threat, slowing them up even more. The combined Russia/UK pressure, with this American help, is what you’re hoping buys you 1-4 extra rounds to have America smash Japan. It costs you about 1 and a half American rounds of production to do it, and that’s early game production, the most important production. Worth it? I dunno. I’d need to theory-craft for hours on this, or just play a couple of games.
My guess is that this whole American fighters in Moscow thing, either going through the Atlantic or Pacific, is an alternative strategy that is non-optimal. Very possibly somewhat effective since you’ll be shaking up your opponent who isn’t used to it, but at the end of the day, non-optimal. But fun wins out over “perfect” play any day.