Meh, really, it simply depends on the Axis’ focus on any particular game. It looks like Russia only gets a crappy +5 NO once it is at war with the Axis, so compared to pissing off the Americans, war with Russia (only) would be a much more palatable option for Japan. 18 land units, while it is quite a lot, would not be expected to be massed together right at the border with Japan, so they could be picked off a bunch at a time, and definitely doable if Japan gets a major industrial complex in Asia (which it probably will in most games). Plus, Moscow it so far away that sending reinforcements east is probably not Stalin’s top priority, so Japan could hold onto the Russian IPCs for a long time, gaining back whatever the cost was in killing the units over 3-4 rounds.
Also, it is subterfuge to some extent; forcing the Soviets to respond to threats on their eastern front would pull some attention (and troops) away from the western front, which would help Germany and Italy pull off a successful Sea Lion AND Barbarossa in turn. Kinda of like not letting your left hand knowing what the right hand is doing. However, with a Sealion going into action on Round 3 or 4, Japan may be forced to focus entirely on eliminating India so that the UK is incapable of sending reinforcements anywhere else.
I’m wondering what Italy will be required to take in the Med to stop UK air from reaching England. Definitely Gibraltar and Malta, and all of the African coast, excepting Egypt maybe. Italy has its hands full. Maybe it would be best for Germany to ditch Sealion and just go after the Med early and take what it can get?