I have yet to see a J1 attack on Russia in any game of Global A+2, though it has seemed interesting lately as something to try out, for several reasons.
First of all, I would only ever attack Russia if it amassed its 18 inf in Amur, and only if I planned to declare war on the Allies on J3. With 10 infantry stationed in Manchuria/Korea and two transports within striking range (as well as supporting BB/CR and aircraft) the Japanese could just about annihilate the Russian stack in one round of combat. With this force destroyed, there is nothing stopping Japan from sending a few units westwards to grab the Russian territories. They’re not worth much, but it would allow the Japanese player to about double its territorial gains during the first few rounds when Japan is only fighting China for 1 IPC a territory, and it doesn’t bring the Western Allies into the war. That little mech the Japanese start with could be very useful in eastern-central Russia.
This move would also prevent the mid- to late-game reinforcement of those 18 inf to the Western Front, where they could have a very decisive effect on the war Russo-German war. Russia could reinforce China in turn during the early turns, but a well-planned strike by the Germans would make every infantry sent eastwards a very tough decision for the Russian player.
Of course, I’ve never tried this strategy and don’t know if I ever will, since Japan has so many strategic options and can easily spread its forces thin.