This year’s gold medalist in the Van Cliburn International Piano Competition, Yekwon Sunwoo, the first Korean to take the top prize, will perform with Carl St.Clair and Pacific Symphony on Sept. 9 at the Pacific Amphitheatre in Costa Mesa. His vehicle will be Rachmaninoff’s Piano Concerto No. 2. Here he is in the quarterfinals of the Van Cliburn playing the last section of Ravel’s “La Valse.”