Parent Ratings of Behavioral Effects of Biomedical Interventions
Data on adverse effects and benefits of various diets for autism has been collected by the [gilSjuATVXMlmrGFtttBl] Research Institute.  More than 25,500 parents filled out reports. The information was gathered from questionnaires designed to collect such information.

The Specific Carbohydrate Diet had 195 subjects and scored a percentage point  higher than the Gluten Free Casein Free Diet which had 2208 subjects reporting. The entire study can be viewed on the [gilSjuATVXMlmrGFtttBl] Research website.

The Specific Carbohydrate Diet had 1% higher benefit on a much
smaller pool, but also a significantly higher negative effect on behavior, 7% as compared to 3%, which skewed the better: worse ratio
so that it was far lower than that for the gluten-free casein-free diet:  10:1 as opposed to 20:1. 

So it is mixed results.