That really wouldn't prove anything, honestly. There are way too many variables (outside factors such as environment and exercise, and internal factors like hte cat's DNA).
Sorry to be blunt, but your logic is seriously flawed. Negative evidence proves nothing. By the same token I could say, "My dresser is an alien repellent. Proof: There are no aliens in my apartment" ("raw diet prevents heart disease. Proof: My cat doesn't have it and that one over there does").
To get any reliable data would require an experiment where variables such as how much exercise the cats get to what kind of air fresheners the owner uses, etc.
Of course we can use anecdotal, informal evidence to support going raw, and cases of individual cats (such as hoofmaiden's cat with IBS) is at least somewhat valid. But to compare feline health on a cat-to-cat basis isn't fair or accurate.