Yes, that's what happened with Krypto. After nearly 2 years, he had begun gaining weight rapidly. It's got to be the new food. I was concerned that he'd feel hungry when we cut it back, but he didn't/doesn't.
I like duck foods, also. We'll have to see what other foods this company makes in the future, since he's doing so well on it, and I'm assuming/hoping that Hira will do well on it, too.
I'm glad that Thomas is doing well on the Iam's food. I've only had one cat on Iams. That was Ananda, the dilute flame point Siamese that showed up here in the rain one day. She literally picked at all foods, but liked Iams. I don't even know how I found that up, since I never bought it. But, once I did learn that it worked for her, and that she would eat it, I had a small bag just for her.
Being a hairball formula sounds familiar. I don't know, though, if it was a sensitive stomach formula as well. That was some time ago. In any case, I'm glad that you can worry about Thomas less by feeding it, and that he feels better on it.
Love All Life, Thank You For Posting! :)