Let’s break it down into pure numbers, using 2nd edition.
Let’s assume a fairly standard J1–attacks on 35 fleet, 37 fleet, Philippines, Borneo, Kwangtung, Yunnan, Hunan.
So first let’s add up the ipcs of air and naval units that are likely to be destroyed J1.
American dd and ss in 35=14 ipcs. American fighter=10 ipcs. It costs Japan 2 inf average to take Phi, so the 2 inf for 2 inf are an even trade. That’s +24
British BB is worth 20, and typically Japan loses a fig to take it out. That’s +10.
Borneo is worth 4 ipcs. So that’s an 8 swing (Japan gains 4, UK loses 4). That’s +8
Kwangtung is worth 3. That’s a 6 swing. +6
Phi is worth 2. That’s a 4 swing. +4
Fico is worth 2. +2
There’s also the potential for a 62 attack (dd on dd) that does even more damage if it works.
That’s +54. So what about the countervailing numbers? Japan misses out on the +10 bonus. Americans will get +20 in bonuses. Anzac will get +10 in bonuses.
Axis seems to come out ahead. It’s closer to even if Axis gets diced at 37 (and loses a fig + cruiser) or diced at Yunnan (which happens occasionally because w/o the bombers you only have 97% with greater average losses). Or if the sz 62 attack fails.
In terms of meta-strategy (in a non Sea Lion game), I think Axis gains more strategic momentum by declaring J1 then they lose in the Atlantic with USA coming into war. For starters, Axis can build minor ICs in French Indochina and Kwangtung on J2…better locations for ICs then are available otherwise. They have a head start on securing the islands.
J1 is significantly less attractive in BM3 due to increased USA bonuses, perhaps to the point of being sub-optimal. But it’s still playable.