A group of Benedictine nuns and a union pension fund want to break up banking giant Citigroup (C), saying it's underperformed since the 2008 financial collapse, and is at risk of greater regulatory scrutiny and other issues that could jeopardize shareholder return. "Citigroup boasts many attractive attributes, but remains burdened by excessive complexity, as well as the stigma and risks associated with being named a 'too big to fail' institution,"