Another difference: the lower cost of bombers combined with the defenseless transports. If Allies go KFG then Japan should buy 1 bomber each turn and send it to Europe in two turns (using conquered Egypt or Trans-Jordan). A large japanese airforce in Europe can make it really hard for the Allies, because Japan plays between UK and US and can take advantage of any openings left after the UK fleet has moved.
You know, I had to laugh at this. I had posted the subject of limits on cooperation b/w the Western Allies and Russia. Being the history nerd that I am, it never would have occurred to me to send Japanese bombers to Germany (and now that I know, I still couldn’t get myself to do it)!
Historically it might make no sense for this to happen (although there were examples of German/Italian subs/surface raiders operating on the Pacific and Indian oceans) but the game isn’t meant to be a faithful representation of WW2 (if so the starting production levels would be grossly unbalanced towards the Alllies).
You may choose to play it so and create your own house rules to approximate it as close to reality as possible but with out of the box rules there’s nothing to stop you or your opponents to do so.