Pro/Con is about trade offs. So… if Japan does NOT attack until J3…
Pros: No U.S. in the war. They earn 20-25 ipcs less, are limited to 3 units per factory, can’t move units away from U.S. shores.
Japan can focus on China (and Russia, I guess, if that’s your thing. Germany taking London is easier since no U.S. reinforcements reach London before Sea Lion.
Con: Japan will earn less income. The U.S.’ 20ish ipcs will still be earned, but by UKPac and ANZAC, which is a more immediate issue come turn 3/4 than the U.S. No factory on FiC until J4.
For Japan’s income: No attack, you earn an extra 20ipcs from a national objective, but only 10-12 ipcs from taking Chinese territories (combined ipcs from J1 and J2). J1 40, J2 43, Then on J3 you’ll only earn around 40ipcs. Maybe 45ish.
With a J1, on J1 you earn 40ipcs (even), UKPac is -7ipcs, U.S. +20; J2 you earn 55-60ipcs (now +12-20ipcs over a J3 attack), UKPac is -14ipcs over 2 turns, U.S. is +40. J3 you earn 60+ ipcs (+30-40ipcs over not attacking on J1) while UKPac is -24ipcs and U.S. is +40. ANZAC will also earn 5 extra ipcs if you attack, assuming if you don’t attack they’ll take 1-2 money islands.
So Japan’s income goes up enough to about cover the extra 40ipcs the U.S. gets, but UKPac will earn 20-27 fewer ipcs (plus you can bomb them on J2).
/my math assumes Japan takes J1 Philipines, Borneo, Hong Kong, FiC and 3 Chinese territories, J2 you take other 3 money islands; J3 you take Malaya