The cynic in me thinks the science diets and expensive brands are specifically designed to liberate cash from naiive but otherwise well-meaning pet owners wallets. I'm not sure there are any 'experts' who haven't been 'bought' either it would be an interesting investigation for Choice.
Meine katzen eats Whiskas dry food and whatever in tins that is on special but still seems reasonable quality. Sometimes the younger members of the family quietly put treats in the trolley, and sometimes the single serve meals are on special enough to make them an option, but never at full price.
Whenever we cook a roast, particularly chicken, mini-puma always seems to get some for 'loudness control reasons' only (even if he was an 'outside cat', we'd still hear the little #$*& through the closed door ) - thankfully after said feast he'd be no challenge for the cat detector van ... (unuashamed Monty Python reference).