Imp Impac Mortgage Holdings has been building up its retained portfolio as a strategy to sustain earnings in an evolving market in which loan production levels are expected to taper off. Impac, a publicly traded real estate investment trust, has carved out a profitable niche in the Alternative A mortgage market. Analysts at Sandler O’Neill rate the firm’s stock a buy, estimating that investors could earn a total return of 23.8 percent if the stock