Pretty sure your S&D regenerates at the same speed no matter what your S&D level is at. I thought your prosperity (growth on fief tab on strat site when accessing from c-rpg site) generates that full amount in 24 hours. So if you're at 240 prosperity, you will gain 10 S&D every hour. If you were at 480 you will gain 20 S&D every hour, etc.
The only thing I know that prosperity affects otherwise, is the price of goods (buying or selling) when below your prosperity mark. If you are below the prosperity mark it will start raising the price if you're a low price/buy fief, and start lowering the price if you're a high price/sell fief. Once you get over prosperity mark it goes back to normal price.