I don't know if Persian's have specific nutritional needs, but in general Innova and Solid Gold are considered tops among the premium food brands. With Innova, you'll will notice a change in their coats; they'll become super soft and silky. I just started my cat on Solid Gold, so I can't say what effects it might have but the ingredients are incredible. It's the only one I know of that includes Menhaden Fish Meal, which is supposed to be good. I'm also feeding her Dick Van Patten's Natural Balance, which differs a bit from other brands in that it has duck in it. Other great brands include Felidae, Flint River Ranch (or something like that), and Chicken Food for the cat lover's soul.
Any of these can be mixed with your Royal Canin. These are all described as premium foods, which stay away from by-products, dyes, fillers, etc. Each cat is different so you have to see what they will eat and which foods they will thrive on. In my personal opinion, a raw diet that is consistantly balanced with the necessary vitamins is the best food for companion animals.
Felidae
Innova
Wellness
Wysong
Solid Gold
Flint River Ranch