I am currently reading Figes "A People's Tragedy" again. While it usually is rather weak on the military side of the Civil War, I stumbled upon some interesting information:
1) The SR rebels in Moscow which arrive early in the Grand campaign: Current OOB: Popov + 1 element of Militia (Moscow Anarchists) + 1 element of volunteers (Moscow Rebels).
According to Figes (p. 634): the SR rebels actually outnumbered the Bolshevic garrision in the city: they had the 2.000 well-armed and trained men of the Cheka Combat Detachment at their disposal (strangely enough that Cheka detachment was formed exclusively of Left SR members). At the same time the Bolshevics had only 700 men to defend Moscow. The bulk of the Latvian Rifles (the only loyal defensive force available) had left for the outskirts of Moscow.
In game terms that means: The Moscow SR rebels should be regular infantry, if not elite. At the same time the garrision of Moscow is much bigger than it historically was.
2) Siberian Commanders: are way to good in this game. I can understand that the Southern White had some first rate commanders but with few exceptions that wasn't the case with the Siberians (many of them were young and/or lacked front experience).
Take for example Popov: the man had Moscow in his hands but instead of taking the Kremlin, the SR leader waited idly until his chance had passed. In other words, he lacked aggression and initiative. In game terms one would expect a 2-0-0 general not a 4-1-1 one.
It might be worth considering reducing Siberian commanders statistics across the board: make the average general 3-1-1, 3-0-0 or even 2-0-0.
3) Allied war supplies: I found some numbers (Figes, p. 652): in the first six month of 1919, the Kolchak regime received 1 million rifles, 15.000 machine guns, 700 field guns, ...
In game terms: Perhaps the Siberians should receive some artillery units for free in addition to the tanks and supply trains they currently receive?
I know that some of this goes against my own words stating that the Siberians are overpowered. But that issue has been adressed in the latest patch (perhaps in a slightly crude manner, but effective nevertheless).