I am a grain free fanatic! I feed my shih tzu/poodle mixes Taste of the Wild and they do so well on it. They're coats are super shiny and soft, their skin isn't itchy anymore, no more ear infections, and no more tear stains.
As for feeding less, I was feeding 1/2 cup 2xday and with the TOTW, they get 1/3 cup 2xday. The first grain free food I fed was Orijen and I was feeding 1/4 cup each meal because Opal was putting on soooo much weight. It's very rich, but I would look at those few pieces of food in their bowls and think "that's not right".
You really can't go wrong with a better quality food and with grain free you can feed all your dogs the same one without worrying about a puppy formula or a senior formula, etc. Do you free feed or feed meals? No matter what you decide on, feeding at least two meals a day will help them feel fuller.
Like smrobs said, look at the ingredients and the first one listed should be meat, not corn.