Here is a data point: 40 Percent of Fortune 500 Companies Founded by Immigrants or Their Children: http://www.forbes.com/sites/stuartanderson/2011/06/19/40-per... And because immigrants represent a minority of the people in america (12.6% as of 2007 [1]; and when adding their children this is likely still less than 40%), we can deduct they create more businesses per capita than americans.
[1] http://books.google.com/books?id=aaimTNHDzZYC&pg=PA32...