Subprime borrowers are paying rates that are closely linked to their repayment risk and reaping benefits when they choose to accept prepayment penalties. Those were the key findings from two studies commissioned by the National Home Equity Mortgage Association. The studies, conducted by researchers at the University of Virginia, analyzed the characteristics of more than 950,000 first-lien mortgages originated last year in a database compiled by law firm Sirote & Permutt, an Alabama-based firm