As someone who almost always ends up playing as Japan - I would have to agree with the earlier poster who said he welcomed an R1 attack against Japan.
The notion that depriving Japan of its AAP40 J1 attack I think overlooks the very real possibility that in AAG40 Japan may be better served on a J2 or J3 attack instead - if only to prevent the US from entering the European conflict early. Remember, a Japan DOW on UK/ANZAC essentially frees the US to DOW Germany/Italy. Why would a prudent Japanese player do that in round 1? I’m not certain its worth getting the US in the game that quickly.
Divide and conquer seems a more prudent approach - and if Russia wants to serve its Asian territories up early, then as a Japanese player, Im happy to take them on, while also reducing the Chinese threat all the while positioning for the inevitable strike against the UK/ANZAC.
So having the opportunity to permanently remove the USSR infantry threat early on, seem to me may actually free Japan to expand farther and in stronger force in subsequent rounds than they otherwise would be able to if they had to garrison forces on the defense in the north.