There is currently a study going on through NIH looking into the prevention of Type 2 diabetes. Diabetes Control and Complication Trial, which looked at management of Type 1 diabetes. The trial proves that glucose control mattered in the limiting of the diabetes complications. You want to target 80-140 premeal. It's not feasible to prove what's the maximum before complications begin.
Activity helps with weight management.
It also improves the body's ability to use insulin. People tend to feel better overall when they are more active. This helps with some of the stress issues related to diabetes. Increased activity burns up more glucose. The muscles need glucose for energy.