Republicans are broadening their investigation to include the Justice Department.
Advocates both for and against the Marketplace Fairness Act said the study helped their side.
The IRS gave $1 million to 1,146 staffers who hadn't paid their taxes.
Rep. Sam Johnson is drafting legislation in response to an IRS audit released this week.
IRS staffers delinquent on their taxes got more than $1 million in bonuses in recent years.
A report from a liberal group says the most prominent chains slashed $232 million off their tax bill through a tax break for stock options and other performance pay given to executives.
The IRS has absorbed about $900 million in cuts since fiscal 2010.
The securities advocate wants "transitional relief" for banks and other businesses.
Democratic senators said the findings underscored current problems with the tax code.
The IRS also got fewer visits to its website this filing season.