I think you really sound like you are doing everything right. There are some children who are very sensitive to chemicals in food. This may really help. As a special education teacher and mother of a very "energetic" child, I know it can.
One thing to keep in mind. Children with sensory issues: ADHD, Autism and sensory processing disorders are often more sensitive to these chemicals. While some of these things can be diagnosed and helped early on, quite a few need to play out and won't really be able to be addressed until the child is in school, especially ADHD.
My advice, keep doing what you are doing. You sound like you are a fabulous advocate for your child. One thing I have learned with my own son, not only are there certain foods: sugar and any artificial sweetner(Splenda is the worst)that affect his behavior, his schedule is his biggest predictor to behavior. I have found a very strict schedule helps him. A set bedtime with more sleep allowed then a person would expect for a child his age as well as predicatability in his schedule. Anxiety over situations or the unknown seem to bring his behavior to the surface as well.
Rememeber, you know your child better than anyone. If you are not getting the answers you need from your pediatrician, there are others out there. Maybe you want a second opinion. Also, your school district may be able to offer suggestions if you call the Early Childhood office.
Good luck. I can empathize with your situation.