I have not read the study yet. It is at http://aer.sagepub.com/content/early/2013/11/25/0002831213513634.full.pdf+html . If it is true that kindergarten is too easy for kids in general, then KG is much too easy for many gifted children.

http://blogs.edweek.org/edweek/inside-school-research/2014/02/kindergarten_is_too_easy.html
Study Finds That Kindergarten is Too Easy
By Holly Yettick on February 13, 2014 12:35 PM
Education Week

Kindergarten might be the new 1st grade but it is still too easy. A forthcoming study in the peer-refereed American Educational Research Journal finds that students make bigger gains in reading and math when they learn more advanced content such as adding numbers and matching letters to sounds. Yet kindergarten teachers spend nearly twice as much time on basics such as alphabet recognition and counting out loud. Study authors Amy Claessens, Mimi Engel and Chris Curran found that the majority of kindergartners already know how to do these things when they start school.

"If you teach kids what they already know, they're not going to learn as much," said Claessens, an assistant professor in the Harris School of Public Policy at the University of Chicago and the mother to a kindergartner. "I would go even further and say more time on basic [content] is actually harmful to kids particularly in mathematics. In reading, it is neutral, but math is negative."

In a paper published last year in the peer-refereed Educational Evaluation and Policy Analysis, Engel, Claessens and Maida A. Finch found that even students who started kindergarten lacking basic skills made bigger gains when teachers emphasized advanced material. The American Educational Research Journal findings add a wrinkle by suggesting that kindergarten students learn more when taught advanced content, regardless of whether they have attended preschool or come from low-income families.

Academic Content, Student Learning, and the Persistence of Preschool Effects is based upon a large, nationally representative sample (ECLS-K) representing more than 15,000 students who started kindergarten in 1998-99. The article, which you can read in full for free for the next month, is the most frequently viewed piece on the American Educational Research Journal's website, even though it was just posted November 13th and has not yet appeared in print. (This is relatively rare since it often takes academic articles years to wend their way toward most-viewed status.) Claessens speculates that one reason that the findings have received so much attention is that they have some pretty interesting policy implications.