We did a study in the mid 1980's comparing an ACT protocol for weight control with a "best of breed" CBT protocol in a 12 session group format. The results were pretty good for both of the interventions, but not different. This was early in the research program. Self as context, contact with the present moment, and values were all less emphasized than they are now.
Jason Lillis' dissertation in 2007 showed that a brief ACT workshop was able to reduce weight related self-stigma and increase quality of life. Even though the intervention did not target weight loss directly, there was an average of 5 pounds lost and maintained at follow up. Check out the publication for the main study and for the weight-specific AAQ.