Here is a remarkable quote from an IMF study:
there is remarkably little evidence in the historical data used in our paper of adverse effects of fiscal redistribution on growth. The average redistribution, and the associated reduction in inequality, seem to be robustly associated with higher and more durable growth.
This is contrary to the larger part of economics history in which growth and inequality were seen as trade-offs. The IMF has earlier published studies with similar results.
Too much inequality can be bad for growth. We have argued some time ago that the rise in inequality has been an important factor in producing the financial crisis, as median wages stagnated and lower incomes started to borrow more to share in the prosperity.
The rise in inequality shifts income from low to high savers which drains demand from the economy (Larry Summers secular stagnation). Central banks then reduce interest rates to keep growth going as inflation is no danger, which leads to increased borrowing, indebtness and possibly asset bubbles.
Does that mean that we should start redistributing left right and center? No, it probably matters a great deal exactly how it is done, and in which context (rich or poor country, etc.). Inequality matters because it provides incentives, which too much equality could blunt.