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.
Personally I feel if the cat doesn't just fit in the household and you've tried everything, your better off finding another home. You don't want to resent him or dislike him. I have had cats that just did not work out in our house, and I found wonderful family members to take them in and they...