I don't know if this is going to help you or not, but this is a topic near and dear to my heart. "Good" cat food is not what cats should be eating. Cats are carnivores and if what you are feeding her is a mix of rice, corn, wheat and meat by products she's getting too many carbs. cats can do well on very high protein diets (like 100%)
Over the years I have lost more than 1 cat due to problems that i believe are caused by the commercial cat food diet. I even switched to the expensive brands and they still gained too much weight!
So when I got yet another "Chowhound" easy keeper cat; she was overweight before she was a year old. Rather than watch her turn into a ball of blubber and then watch her die young from a food related disease, I started feeding my cats 50% raw meat. My oldest cat just won't make the switch, but Enza, my chubby cat and Aggie happily scarf down ground chicken (whole chicken, including bones and hearts and livers) every day. I also feed them dry cat food because they're used to it. I'm hoping that for my cats, at least this will be an answer.
The science diet (or whatever it is) is probably a lot better for your cat than what you were feeding her, and I hear cats do very well on insulin.
Good luck.