I've been thinking a lot about the merits of the (highest income / median income) ratio in creating a society that is a little more resistant to eventual inequality. Rather than setting absolute tax brackets, what if we define tax brackets by this ratio? That way, it will be in the interest of the highest income earners to raise the median income.
2. People can move. Labor is more geographically sticky than capital, but it is not infinitely so. I doubt you'd ever succeed in getting Singapore to stop being the capitalist's paradise through foreign policy.