It really depends on the cat. I don't think I've ever heard of someone switching from a higher quality food to a lower quality food and saying that their cat's coat was shiny and that the cat felt healthier overall, but that comment is frequently made when they "upgrade" their food choice. I think a big part of what make the more expensive foods "better" is the consistency in the ingredients, especially the protein. By-products are thought of as bad because we never know what they might include--only that it isn't the regular meat. Corn frequently causes allergies and digestive upset, so I steer clear of foods with corn in them. I don't know whether the better foods always cause a longer life. They do seem to improve coat condition, which is one of the easiest things for us to judge, since we can see it and feel it.