It is an topic with many opinions and questions: Should cat food have high levels of protein? Does that protein cause kidney disease? Do cats with kidney stress need low protein foods?
My opinion is that cats are obligate carnivores and therefore, are made to eat protein (and fat). However, the TYPE of protein and quality of that protein determines how that protein is assimilated and used. Cats, being carnivores, are made to eat protein from meats and not plants. However, many dry foods use plant proteins (corn, soy, etc.) which do not have as high of a biologic value (they cannot be used as well) as meat proteins. Meat proteins also contain required amino acids that plant proteins do not (taurine, for example, is only found in meats). So why is it that many pet food companies and some veterinarians say that excess protein is hard on the kidenys and is bad? Well, it comes down to this... when you include soy or corn protein (or animal sourced proteins like lower quality by-products and by-product meals), it becomes hard for the kidenys to assimilate that protein and deal with the waste that cannot be assimilated. Another issue is that low-quality meats and meat meals have more ash than high-quality meats and meat meals. This is also hard on the kidneys and will likely lead to problems in the future. In addition to all the above, protein delivered in food that is more natural to a cat (more similar to what they would natually eat) and has a high water content (cats have a low thirst drive) is much easier on the kidneys.
Many "senior" foods have less protein because as cats age, if they have been fed a lower quality food, they will have kidney damage and these foods are thought to minimize future damage. HOWEVER, senior cats actually need MORE protein in their diet due to aging processes that can cause muscle wasting.
So, what is the best way to feed a cat the protein they need without compromising kideny health? Feed your cat(s) the way they are meant to eat, with a food that has sources of high-quality protein at reasonably high concentrations. Dry foods are OK if they are of a high quality (and I feed one as a "base" diet as well) but, if possible, you should really include a canned or other wet food (I feed raw as well as canned) to your cat's diet to help maintain their kidney health for many years to come. However, if your cat has a medical condition, always talk to your veterinarian before switching diets.