I believe split VC conditions for each theater (8 in Europe, 6 in Pacific) is already quite a strong motivation for US to pay attention to both theaters. Maybe US players don’t get as much short term reward by splitting its resources to both theaters as they get by focusing on a chosen theater. But dedicated focus to a single theater even though bringing fast progress in the chosen theater will most likely lead to lost war in the other theater.
In my current test game with US+9 (http://www.axisandallies.org/forums/index.php?topic=35735), I tried KJF. I dedicated almost all US income to Pacific and was able to capture Tokyo in US9 but I will likely lose the game pretty soon :(. Even at +9 to US, KJF is not really overpowered and probably not optimal at all. Also I believe Japan could have prevented fall of Tokyo, just somehow under-defended it, well they don’t need it to win the game.
I am currently playing three test games with extra US income and I believe we have not observed any strange artifacts felt by the additional US income, so overall I like it. In 2 of the games, it is first time for me to face the dark sky strategy (Germany having 27 bombers in 9th turn), and I don’t enjoy playing against that strategy much, it feels so unrealistic and misused. It feels that Germany is overpowered by massing bombers in such an extreme way.
As for the balance of US economy bid, in all of my games, +10 seems a bit low, especially if Italy does not get “tarantoed” then Italy is also having about +10 bonus compared to what it is normally used to have with a pre-placement bid in MED. I will try +15 to +20 range next I think. I will start a google sheet soon, where we can summarize results and experience from all played test games.