I would ask your vet straight up if he feels this is related to diet. If your cat was diagnosed with cystitis, or flutd, and/or having crystals, then what you are feeding is very important. I can not find an ingredients list for the Kirkland brand cat food, so I can't form an opinion on it. I do not recommend Whisker Lickens treats...here is an example of why...the chicken and liver tartar control ones has this ingredients list (this is just a partial list):
"Ground yellow corn, wheat flour, soybean meal, corn gluten meal, chicken by-product meal, beef tallow preserved with BHA, liver and glandular meal, brewers dried yeast, phosphoric acid, calcium carbonate, dried cane molasses, poultry by-product meal, animal digest.."
Corn is a potential allergen for cats, and by-product meal, animal digest are not quality sources of protein. Your main ingredients in this particular treat are all carbs, not what cats need, and BHA is a much debated preservative, felt to not be safe for man or beast by many.
I would get some more information from your vet - you need to know if crystals (struvite or calcium oxalate) are involved or any chronic inflamation as occurs with cystitis. Here is a good overall article:click here
For sure, I would recommend you consider feeding a canned diet instead of dry if possible, and doing what you can to increase your cats daily water intake (get a pet water fountain is one good way to accomplish this)...but exactly what kind of diet would be best for your cat, depends on what the vet feels is going on.