A summary of this paper is now available:
Who Benefits from Education for the Gifted? In Does Gifted Education Work? For Which Students? (NBER Working Paper No. 20453), David Card and Laura Giuliano report that full-time classes set up for gifted students don't raise the achievement of gifted students, but have large positive effects on non-gifted high achievers in those classes - especially on the reading and math scores of low-income high achievers. The authors conclude that establishing "a separate classroom in every school for the top-performing students could significantly boost the performance of [these] students in even the poorest neighborhoods," without harming other students or increasing school budgets.
