This may all depend (in the long term) on the matter of UK/US units being allowed on Soviet soil. If yes, as soon as Russia IS at war then the old Karelia strategy will be the obvious Allied approach, and it’ll just become a superstack race. Why have idle units stacked in England waiting to risk a D-Day crossing when you can constantly feed them into Karelia/Poland and have them fighting Germans every turn as well as reinforcing the Soviets and setting up a 3 vs 1 (or maybe 3 vs 2) push on Berlin?
If no, then the WA will need to protect those 3 vital convoy routes into Russia to keep Stalin afloat. Personally I much prefer this rule, especially if there is a sensible restriction on Japanese routes to Moscow, removing the need for the WA to push maximum units into Russia in order to balance the JTDTM.
In other words the restriction on Japanese progress through Siberia is balanced by the fact that the WA are restricted to a (limited) cash aid program for Russia and can concentrate their military focus on more historical objectives in WE and the Med.