You can find the studies by Keith Gaddie and Charles Bullock here on the American Enterprise Institute’s page for its “Project on Fair Representation.” You can also access on that page (direct link not available) a report on the data by Edward Blum and Lauren Campbell, Assessment of “Voting Rights Progress in Jurisdictions Covered By Seciton 5 of the Voting Rights Act.” That report begins: “After collecting and analyzing extensive data on the state of voting rights in jurisdictions covered by section 5 of the Voting Rights Act, we conclude that there is no longer sufficient justification for the preclearance mandate. Greatly increased minority voter registration rates, minority voter turnout rates, and number of minority elected officials all indicate that the aim of the act has been fulfilled–voting rights and representation for minorities have solidified and section 5 should be allowed to expire.”