Hi - in your case with your cat having kidney issues, your cat would want to be on the lower protein food. However, from the reading i've been doing most experts seem to say that higher levels of protein are better. My cats have been eating Royal Canine weight control - it has protein of 38.5 % (42.1 if you look convert to dry matter). I had also tried their vet diabetic food to help my cats loose weight - it is at 44 % and 48 % if you look at dry matter. However, due to my obvious concerns about corn gluten in these products I've been evaluating other products. I've currently moved my cats to 50 % canned (Felidae, Wellness) because, as you mention, most kibble is in the 30's for protein. There are a few exceptions - CORE, EVO - however, those are so high in calories that I can't put my cats on those either. I've done reading from MANY sources and there really seems to be an overwhelming concensus that canned is better for cats health, but we frequenly choose dry for convenience sake. There is a web page that does an excellent job of summarizing the research in this area:
http://www.maxshouse.com/feline_nutrition.htmHere are a couple of others:
http://home.earthlink.net/~jacm2/id1.htmlhttp://www.yourdiabeticcat.com/