Billionaire Michael Platt, who decided to relinquish his external clients and about $7 billion in fee paying assets two years ago, is on a roll. Free to take more risks and sweeten returns with borrowed money, the former hedge fund titan led his private investment firm to a 54 percent gain last year, according to a person with knowledge of the firm. The outsize return came on top of an almost 50 percent jump in 2016, BlueCrest Capital Management’s first year of trading with Platt’s and his partners’ money. A spokesman for the firm declined to comment.