This is the top 10 fief's S&D growth for EU :
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Notwithstanding the duration of the strategus round 4, the unlimited and linear process of increasing your prosperity (1 Production Point invested, equal to +2 prosperity) has produced extreme quantity of goods thus silver, to a point where silver does not matter much, except that it is still necessary to round it up and go buy stuff.
Shouldnt prosperity be soft/hard-capped ? Anything that would slow down or even limit the economical power of fiefs would also give more incentive to manage your S&D appropriately.
The first months of strat 4, everyone was guarding their S&D like its their life, it created drama and economical wars. Now that everyone is Cresus, only troops matters: S&D unlimited growth over time has successfully removed a part of the game.
My personal ideas:
Village max S&D growth : X (average, but hard to defend thus hard to depend on)
Castle max S&D growth : Y (low production, incentive to become armouries instead)
City max S&D growth : Z (high, because cities are rare)
Formula for PP cost to Prosperity bonus should become a logarithmic function, like the trade distance bonus (read: PP cost for 1 prosperity increases if you have more prosperity, making it hard to reach very high values).