Redshirting: Article III

This one shows little benefits and does mention the downsides of redshirting:

Redshirting and Early Retention: Who Gets the "Gift of Time" and What Are Its Outcomes? (M. Elizabeth Graue and James DiPerna, "American Educational Research Journal", Vol. 37, No. 2, 509-534 (2000).)

The concept of immaturity is used most frequently as the rationale for delaying kindergarten entry of for early retention. [...] However, interesting findings from other work make us question the common wisdom of redshirting for the immature child. Both Byrd et al. (1997) and Zill et al. (1997) have noted that children overage-for-grade, regardless of whether they are redshirted or retained, are more likely to have social and emotional difficulties later in their school careers. Using samples of children from ages 7 to 17, this pattern was particularly prominent for middle and high school students in the Byrd et al. study. The strongly held hope that the gift of time will allow children to fit into the demands of schooling is questionable on these grounds.