I agree with Pancake and kcdzim about (not) attacking with the siberians.
I think it is better to wait attacking Manchuria/Korea untill Japan is comitted in SE Asia. (and cannot counterattack your siberians in 1 turn) OR have Japan leave a large defensive garrison which also hurt its offensive in China/SE Asia. Create headaches for Japan for as many turn as you can, not sacrifice everything in 1 turn.
On the other hand the Siberians can arrive in time to defend Moscow, especially if the UK has FTR to help defend Moscow. Furthermore, the Siberians add tremendously to the Russian ability to counterattack Bryansk RU7-RU8. They force Germany to focus on nothing but Moscow, leaving western parts of ‘the reich’ more vulnerable.
Sending them all away will result in lost Russian production, but 18 to 21 units (incl. Mongolians) will arrive in Moscow, for a neutral net result until RU10. Keeping them all to oppose Japan results in more production but still less units for Moscow. The increased production cannot replace the loss of 18 to 21 units for Moscow, at least untill RU10.
So I think a RU1DOW is a good thing but what you do with the Siberians is a different thing entirely. and greatly depends on your German opponent and the pressure you will expect on Moscow…