The upgrade by RBC Capital Markets to Outperform and the increase in price target to $18 reflect a strong positive sentiment towards First Horizon's financial health and future prospects. The mention of strong net interest margin expansion and the potential for more aggressive capital return, including a large buyback authorization, are key drivers for the stock's positive outlook. The analyst's positive view on the company's near-to-medium term fundamental outlook, including balance sheet growth and revenue trajectory, further supports the expectation of stock performance improvement.