Some diabetic cats can handle the lower carb dry foods, but some cats are very sensitive to them, even if they are the lower carb ones. This is because they are still high glycemic because of the manufacturing process (all dry food must use a starch to bind it together, or it would be powder). Bandit can eat 9-10% carbs in canned foods with no effect on his BG, but if he eats a handful of EVO dry it shoots him up into the 200-300 range.
Is he also eating canned food? You could try eliminating the dry and see if it helps bring him down. Also, I agree that you might see improvement with Lantus or Levemir. Prozinc is a good insulin and works for a lot of cats, but Lantus and Levemir are a bit better in terms of remission rates and effective control of BG. If you haven't had good control with the Prozinc since July, I'd say you've given it plenty of time to work and it's just not happening.