This paper recently published in Learning Disability Quarterly shows that computer scoring of writing samples performs as well as hand scoring for students in grades 2-5. The full paper can be viewed here: Mercer et al LDQ automated text evaluation.

Mercer, S. H., Keller-Margulis, M. A., Faith, E. L., Reid, E. K., & Ochs, S. (in press). The potential for automated text evaluation to improve the technical adequacy of written expression curriculum-based measurement. Learning Disability Quarterly.