I've been feeding my 4 older cats grain free canned and dry for several years now. Only Ziggy still has a bit of a weight problem. I don't and can't free feed because I have some real piggy cats and the others wouldn't get their fair share. I'm currently feeding Storm,Sky, Pearl, and Ziggy Wellness Core dry and they also get Wellness grain free canned in the chicken, turkey, chicken&herring, and turkey&salmon flavors. Unfortunately Alani & Blaze didn't do well on these foods so they're on a different diet that does have some grains but it doesn't have any corn or wheat.

If you do decide to try a grain free diet I'm sure that you'll notice:
less stool waste
weight loss
more active cats
happier cats
healthier cats
one con is stinkier stools though

Good luck.