Nikkei climbs to new 5-1/2-yr high, ends above 15,000; Sony soars

* Japan stocks seen as overvalued for 1st time in 4 yrs - BOFA Merrill * Sony surges as U.S. fund calls on company to spin off unit * Exporters lead gains on further yen weakness * Higher bond yields weigh on real estate firms, non-banks By Tomo Uetake TOKYO, May 15 (Reuters) - Japan's Nikkei average rose 2.3 percent on Wednesday, breaking above 15,000 for the first time since January 2008, with Sony Corp surging after an activist U.S. fund called on the company to spin off