I find the US/UK vs Japan option (1941) is very viable. Maybe I am misreading what was said before, but it actually seems more viable to me than a traditional kill Germany first. (I think this is because Italy can liberate/take land and Germany can reinforce it with fighters to defend now, a very significant and major defensive capability!)
America may be limited to 52 IPC or less a round if they go Kill Japan first. However, England should be making significantly more than the 20-30 IPC they get in a kill Germany/ignore Japan campaign because of the NO in the Pacific, the non-loss of the NO in the Indian Ocean/Africa/Australia/etc, the increased land ownership in the Asian SE and SE Pacific Islands, etc. Meanwhile, Japan shouldn’t be earning too much either, America could, realistically, be earning near to more than double what Japan is if played right.