I hadn't really considered what would happen if Kingsley ate a human, but theoretically, he would gain their qualities and/or abilities for a day. If he ate a smart guy, he would become smarter. If he ate a dude with fire powers, he'd probably get fire powers. Mind, he wouldn't gain as much smarts or as powerful flames as the original.

Theoretically, of course. I'm certain there are rumors about him and cannibalism, but it certainly hasn't happened before.