Equality, Taxes & Business Growth
To provide some input (which I do not necessarily endorse), the Tax Foundation has used its "Taxes and Growth" (TAG) model to estimate/simulate the effects on USA Economy of Thomas Piketty's proposal that the tax rate for very high incomes should rise to $80$%. 
The results can be found on a report freely downloadable from here.

Their Key Findings are (quote)


  1) If ordinary income were taxed at the top rates of 80 and 55 percent, our model estimates that after the economy adjusts, total
  output (GDP) would be 3.5 percent lower, wage rates would drop 1.6
  percent, the capital stock would be 7.4 percent less, and there would
  be 2.1 million fewer jobs.
  
  2) If capital gains and dividends were taxed at the new tax rates along with ordinary income, the economic damage would be much worse.
  GDP would plunge 18.1 percent (a loss of $3 trillion dollars annually
  in terms of today’s GDP), the capital stock would be 42.3 percent
  smaller than otherwise, wages would be 14.6 percent lower, 4.9 million
  jobs would be lost, and despite the higher tax rates, government
  revenue would actually fall.
  
  3) Although Piketty’s proposed income tax increase may appear to target only upper-income taxpayers, all income groups would suffer
  from the economic fallout.
  
  4) Our model estimates that the after-tax incomes of the poor and middle class would drop about 3 percent if the higher rates do not
  apply to capital gains and dividends and about 17 percent if they do.


The reasons that I say that I do not necessarily endorse the above results is that the report provided, although produced by a "Michael Schuyler, PhD" is not a scientific paper -so we do not know what assumptions and structure does their "TAG" model incorporates.