From Neal Lerner's July 2011 message to WCenter:
Folks, the latest issue (July 2011) of the Journal of Writing Research includes a very impressive study by James Williams and Seiji Takaku of the relationship between college students' self-efficacy, help-seeking behaviors, writing center visits, and writing performance. Here's a link to a pdf of the study:
http://www.jowr.org/articles/vol3_1/JoWR_2011_vol3_nr1_Williams_Takaku.pdf
Here's a snippet to take to your dean:
"The analysis showed that those students who frequently obtained writing center tutoring received higher grades in composition than those who did not, regardless of their ESL or native-English-speaker status. The frequency of writing center tutoring seemed to be especially valuable for the ESL students, who outperform their domestic cohorts, receiving significantly higher grades in composition."