If there's going to be an economy nerf, I'd balance it against ship costs. Top-of-the-line warships in 1940 (for example) shouldn't be pushing the billion (and usually much higher) figure mark. Slashing economy growth numbers by a flat percentage across the board, as well as ship costs, could be a good thing. But as I've no idea how you'd go about balancing this nerf, other than a quaint idea, I don't have much more to say... other than the frankly absurd amounts of money any nation can rack up.