Visa, the world's largest payments network, has shown impressive growth since it went public in 2008, in what remains the largest U.S. IPO by deal size. Revenue and profit growth have hit double-digits every year. Shares have more than doubled, setting a record high on Dec. 20. But Visa (V) is not immune to head winds. While the card giant has managed to keep growing in the face of new financial regulations, its U.S. debit business