Cats with impaired kidney function have a hard time dealing with natural toxins created by the digestion of protein. CKD is not curable so CKD foods are designed to extend kidney function for as long as possible.I know it is the case with dogs, and I'm going to assume the same is true with cats, but the "conventional wisdom" for a long time was to decrease protein in formulas of senior dogs.
Which has turned out to be exactly the opposite of what's optimal.
Older animals need protein as seniors to help maintain their muscle mass (which becomes harder in old age). The newer thinking is that formulas have been reducing protein just when pets need it most (aside, perhaps, during infancy).
My "inner-intelligence" leads me to believe that the rampant kidney disease we see in felines is due (mainly) to the twin problems that arise from feeding dry food. Namely, chronic dehydration (which is very rough on kidneys) and unnatural levels of carbohydrates.
For me, I would not deviate from what you are doing w/o good cause.
You might consider adding some organs to your rations (5% liver, 5% kidney) as a means to enhance the nutrient support (organs are high in vitamins), but otherwise I would stick with what you are doing.
Bill
Under the non-raw section of this forum there's a link of a study which suggests that raising cats on food which is high in phosphorus may cause kidney problems when the cats reaches adulthood.
Another thing to keep in mind is that domestic cats live longer than feral. Consequently domestic cats are more likely to be alive long enough to develop health problem which they were not evolved to have to contend with on the wild.