People need to spot focusing so much on % this or % that in dry feeds, it's not all that quantitative unless that is the sole feed diet, dry feed percentage is factored and based on the feed being a sole diet to provide an average milligrams or grams of whatever the bird needs for a balanced diet, once you introduce 'treats', free range, extra activities or whatever else that % is for all intents is a useless measure...
With that said an average laying layer needs about 17-20 grams of good protein a day, and that is what is important not a percentage printed on the bag unless that dry feed is the sole diet used... Also as said that percentage of a balanced diet is an average for production birds, a bird that is molting for example will benefit with more protein, while an older hen that isn't laying as often could likely get by with a little less...
Also excess protein is easy for the birds to pass, it results in wasted protein and more stinky poop but it's not really a problem for the bird if it's fed excess protein as long as it's still getting a balanced diet...
That said you have to balance the protein intake with the other required nutrients and make sure the birds feed intake for the day is balanced, one could easily feed a chicken three hard boiled eggs a day and meet their protein (and many other required nutrients) each day, but that is not a balanced diet on it's own in the long run...
A thing to watch with the cat food is the salt content
Honestly it's not that much of concern... A lot of 'better' cat foods do not contain any added salt beyond what is in the ingredients, read the label if you are concerned, but in the end the amount of salt in the cat food is no where near harmful levels especially when cut 1:3 or 1:4 into chicken feed... The same with many other 'baddies' in cat food, moderation is a big factor in any diet, especially for essential minerals like salt...
Here is a copy and paste from a previous post of mine on salt toxicity concerns...
Like many of the 'do not feed' things the level of toxicity of salt is not as much of a concern as some pretend it to be moderation is key...
You would have to feed the chickens high levels of salt to cause any real harm, an occasional and moderate dosage of salt isn't any more harmful then a chicken that free ranges in a coastal area where salt is all over everything... It's all about moderation, a little extra salt here and there isn't going to cause any harm, especially if they have plenty of fresh water to wash it out of their systems...
It's when you have high levels of sale and/or limited fresh water or a diet constantly high in salt that you see problems...
This is a horrible scan but it still is readable...
https://archive.org/stream/toxicityofsaltfo00mitc/toxicityofsaltfo00mitc_djvu.txt
If you gloss over it you will see that chickens given 4 grams of salt twice daily appeared to do fine, that is a lot of salt, this is about about 1.5 teaspoons of pure salt a day!
Consider this for a practical measure, your average chicken should consume about 3/4 -1 cup of dry feed a day if it's the sole diet... I suggested about a 1:3 or 1:4 supplement of dry cat food, aka about 1/4 cup of cat food to 3/4 cup chicken feed per day, or about 25 grams (I just weighed it out) of dry cat food per day... Reading the above linked article it suggest up to 8 grams of salt a day causes no observed ill effects... So when you are only feeding 25 grams of dry cat food, unless that cat food is in excess of 33% salt it's likely not going to be harmful, and no cat food has anywhere near that level of salt in it as it would be intolerable, seawater is only 3.5% salt...
Also when I suggested cutting cat food with the feed, it is suggested as a 'stop gap' test to see if the protein level is the problem, if it is determined to be the problem a better long term solution is to simply get a higher percentage balanced poultry feed...