Among the better-performing funds are those that were started in the past several years by former disciples of Steven A. Cohen, the billionaire investor who founded SAC Capital Advisors. One is Jason Karp, whose Tourbillon Capital Partners, a $2.9 billion fund, is up 18.44 percent in its main portfolio, said a person briefed on the firm’s performance. Mr. Karp, 38, who worked for Mr. Cohen for nearly four years, created Tourbillon in 2012. And he is not the only former SAC trader enjoying what has been a long, hot summer for hedge funds. Aaron Cowen’s Suvretta Capital Management, a $2 billion hedge fund, is up 7 to 9 percent in its main funds, said a person briefed on that firm’s performance. And Melvin Capital, a roughly $1 billion firm that Gabriel Plotkin founded last year, is up nearly 20 percent, said a person briefed on the matter. [Dealbook]