In this 2011 PERI paper by Thompson & Leight, “Searching for the Supposed Benefits of Higher Inquality,” the authors review the messy literature on the relationship between economic inequality and economic growth. They propose an alternative approach to this research stalemate.
If you are interested in charting the relationship between inequality and economic growth, urban & regional-level data is your best bet. Only in 2006 did Saez & Picketty pull together (fairly) reliable international comparative data on income inequality. See Saez‘s data and analysis on his Berkeley website.