On the topic of Russia..
I notice that the Cumans like to rolfstomp the Russians every decade or so with their Raid CB. In 1066 the combined levies of all the Russian Princes is about 11k in PB, roughly equivalent to what the Cumans can raise. In practice the Russians have a huge disadvantage due to their disunity; they're hopelessly outmatched in 1066, let alone 50-100 years down the line when gavelkind has made them even weaker collectively and more likely to be fighting each other. The rough equality in numbers between the Cumans and collective Russians indicates to me that they were intended to be about equally matched, which I think is a good target; historically the Cumans fought (and fought alongside, occasionally) the Rus quite often with very mixed results. So I think the Cumans need a nerf; they're too much more powerful than any of their neighbors, in fact they're more powerful than most of their neighbors combined.
Plenty of ways you could achieve this, but I'd like to suggest one in particular that might not be obvious:
(Note: This is from my mucking about with the FVtG map, ignore everything but the Kipchaks, particularly mega-Kiev
)
k_kipchak is held by another member of the Terteroba dynasty, the brother of the Cuman leader in this setup.
My rationale here is that the Cuman-Kipchak federation wasn't a country and it didn't have a single leader. It was a "Khaganate" in the same way the Kievan Rus is sometimes referred to as the "Rus Khaganate"; since Russia is represented as many independent but allied Principalities, why not represent the Cumans as separate but allied tribes? The Cumans in 1066 now have a total levy of ~6.5k, while the Kipchaks have ~4k. They are allied and can call each other to war. Obviously this'd have to be playtested quite a bit, but on the face of it it looks like a more balanced (and historical, or at least historically consistent) setup.
On historicity: Don't have a scanner on hand a show you a more reputable source, but what you generally see in maps of this period is the Cumans (i.e. that part of the federation that had significant relations with the Rus, Hungarians, etc) as a sort of vague blob around Sarkel rather than the entire federation as a coherent entitity. See this:
http://geacron.com/en/?v=m&lang=en&...yyyyy&fi=-500&ff=1500&sp=2&e=0&rp=0&re=0&nv=2
Just throwing some ideas out there
tl;dr - Cumans are OP