The financial performance of UMass Memorial Health Care Inc., the largest employer in Central Massachusetts, fell well below expectations in its recent fiscal year, raising the possibility of layoffs and cost-cutting in 2012. John G. O'Brien, the system's CEO and president, acknowledged that the results have him and his executive team analyzing the business model, a process that could include the elimination of some full-time positions, shedding of some ventures and pursuing changes to employee pension plans.