I agree on cat genetics playing an equal or greater role in health than food. I saw that myself with Trin ballooning on the corn-kibble and Gohan staying perfect. Some cats process things better.
Not to derail the conversation, but I actually consider the Petcurrean Go Fit + Free line to be pretty good food. The gums are at the bottom of the ingredients, and while there are way too many fruits and veggies (and cottage cheese!?), the top 8 ingredients are meat or meat broths. Also it is very highly rated on catfooddb.com and the composition of 55% protein/36% fat/5% carbs is close to ideal. I just fed Olive a sample can and she loved it so I plan on adding it to the rotation.i feed okay wet food (petcurean go, sometimes fancy feast classic pate)