I know that at turtle farms in other countries where they raise turtles for meat they use trout pellets (food for trout). I bet you could get that through a farm or feed store. That would be far more appropriate, even though it's fish. But from what I've found is that that is far more similar to turtle foods than cat foods are.

Again, its not just the protein, but the type. Also cats require high protein throughout their lives. It wont matter what you choose. It's still going to be high concentration of protein made for a mammal's liver.