This study found that, though workers undergoing mental health treatment have lower productivity than either workers without mental health problems or those not receiving treatment, productivity scores improve to near-normal levels after treatment. 

Hilton, M, F., Scuffham, P.A., Sheridan, J. D. et al. (2009). The association between mental disorders and productivity in treated and untreated employees.  Journal of Occupational and Environmental Medicine. 51:9, 996-1003.  DOI: 10.1097/JOM.0b013e3181b2ea30 http://journals.lww.com/joem/Abstract/2009/09000/The_Association_Between_Mental_Disorders_and.5.aspx 

Authors: Michael F. Hilton, Paul A. Scuffham, Judith D. Sheridan, Catherine M. Cleary, Nerina Vecchio, Harvey A. Whiteford.