I agree with knp, though I think I’d use less units against Russia. Attacking Russia doesn’t need to be at the cost of progress in China & India.
My basic stance towards Russia is: if all the Siberians stay to face me and position well (i.e. not in Amur where I can instantly destroy them), I don’t attack J1-J3. If Germany is going to launch Barbarossa, Russia is in for a shock in Europe without these Siberians over there. If Germany is launching Sea Lion, too bad for Japan but the Siberians are here to stay…
On the other hand, if all the Siberians retreat towards Moscow (and if barbarossa is suspected, I believe Russia should) or if only a few, say, ~6, remain east, I will atack. Starting with Soviet Far East, Siberia and Sakha and then further westwards. Activating Mongolia once reinforcement have arrived to kill them too (next turn).
Around turn 4, no matter if all the Siberians are facing me or not, there will be an attack and the Siberians Hunters are either destroyed or become the Hunted (i.e. they flee).