*shrug* I went noble republic with Muscovy last night. Fed Novgorod to Pskov, Tver to Yaroslavl, and Kazan to Qasim (I should have just vassalized Kazan in retrospect) within the first 10 years. In the time it took to get to noble republic (relatively brief), I had fed Qasim all of Nogai, most of the Golden Horde, and some of Crimea. Not to mention grinding Lithuania/Poland into the ground. Expansion is doable, just have to be creative.
Why do it? Well.. Administrative + Plutocratic is actually quite good. The force-limit benefits alone are obscene, especially with additional modifiers from states like Muscovy. Each 25% of additional mercenaries add an additional 3x multiplier to your force limits. With the basic Russian region (Muscovy/Novgorod), you're talking 500k forcelimit possibility in a culturally and religiously homogenous zone that is easy to spam upgrades on.
I would have gone merchant (they can be a little more fun) - but wanted the aristocracy line in this particular game for the tech boost, and other handy modifiers.