If you really want to stick with buying at Walmart, I would suggest Purina Naturals. Of course it's not the "best" but it is a decent food, and I think better then what your feeding and better then the IAMS.
I also agree with that is they eat it and do well on it, I wouldn't stress on it. Many cats thrive on friskies, cat chow, Science Diet, etc.... One of my cats can't handle high quality, he gets the runs from it.