Since the event was created in 2004 there have been 29 participants who have reached the Big Leagues. Currently these players are either established contributors and are on active rosters or recently underwent surgery:
Cardinals RF Stephen Piscotty, Dodgers CF Joc Pederson, Indians RHP Bryan Shaw, A's SS Marcus Semien, Nationals RHP Joe Ross (TJ surgery July 2017), Marlins RHP Kyle Barraclough, Mariners OF Mitch Haniger, Phillies C Andrew Knapp, Reds RHP Robert Stephenson, and most recently Mets RHP Chris Flexen, who made his debut on 7/27/17 and has two big league W's so far! Recently, Elk Grove HS alum J.D. Davis was called up by the Astros, making him #28 on the list and he's been really hitting well with the Astros.
There are plenty of other former event participants who are also legitimate prospects and making noise in their organizations, such as Mariners SS Drew Jackson and Mariners OF Braden Bishop, who could make his way up in 2018.