hi and welcome to our cat forum, first of all if you want to improve their coat and physical health YES you most certainly do need to feed them a much better diet and fast!!!
No wonder they are always hungry, they are starving for nutrients.....most foods are filled with grains and animal by products NOT real meat.
Dry food is a no, no.....its full of chemicals to preserve and its lacking in the much need moisture cats need. dry food has 10% moisture cans have 78%. and cats never drink enough to compensate....this will lead to many health problems mostly FLUTD. or crystals in the bladder/blockage and CRF chronic renal failure.
I will give you a site written out by a Vet....she promotes homemade raw food, however towards the end of her site she does give teh name brands of the best quality GRAINLESS CAN foods on the market.
she does say somewhere on that article that even the WORST canned food is BETTER than any dry food!!
the good can foods are more expensive but cats won't require as much and you'll save $$$ in Vet bills down the line.
http://www.medhelp.org/posts/Cats/GOOD-BRANDS-OF-COMMERCIAL-GRAIN-FREE-CAT-FOOD/show/1442981