The literature definitely shows a positive correlation between diabetes self-management education (DSME from CDE's and RD's) and cost-savings as well as avoidance of complications, hospitalizations, etc both for people with prediabetes and diabetes. Not so much for non-CDE providers (for example, RD's have better outcomes than MDs who try to give patients diet advice): studies comparing usual care provided by physicians to MNT services provided by RDs show that RDs produce better lipid,diet, physical activity, and weight outcomes (Hooker, Williams, Papneja, Sen, Hogan, 2012 pS84).
Here is a sort of literature review or annotated bibliography that I found. http://c.ymcdn.com/sites/www.chronic...tracts2012.pdf
The CDC also has the Diabetes Prevention Program which is shown to really impact people's disease progression.
These are formal programs. This is not the MD telling the patient what to do. This is structured. So there may be some bias in there that patients who go to these are already self-selecting and are more motivated than patients who do not.
So similar to what others have said, the answer is "Yes, but..."