The name of the food that our vet gave us to feed Socks is Purina Veterinary Diet DM (Diabetes Management Feline Formula). It's new and it's a canned wet food, but it has been seen to help manage diabetes in some cats, even without the shots. You can only get it from a vet, so I'm sure your vet could get some for you, even if he/she has to order it. The wet food has been seen to be better for the diabetes control as opposed to the dry food, and it looks like there is plenty of fiber in it. It would be worth a try with Speckles.
You mentioned that she is overweight? I was thinking about this, and if she went on this food, and lost weight...the diabetes may even go into remission. Sometimes weight gain can cause the diabetes, so either way...the food should be a good try! Hopefully you have caught this early enough that it's just in the beginning stages, like we did with Socks.
Give your precious Speckles a cuddle for me. A visit to the vet should help clear up any questions and will let you know if there is anything else going on with her. I'm happy to hear that she seems ok otherwise! That is a good thing!
Do let me know what happens!

Lots of love and **hugs**
Karen