First find out if your feline friend is actually fat. Weight has nothing to do with it. Check out the Purina 9 point Body Condition Score (BCS) here https://www.aspcapro.org/sites/default/files/nestle_body_condition_scale.pdf (it was in an old newsletter?). It is OK for your cat to have a BCS of between 5 and 7 as long as your cat can still jump 1 metre up and has no trouble jumping down. Arthritis stops cats moving and they then become fat and sore, and cats as young as 18 months old can become arthritic and you need to treat that. It turns out that most cats are BCS of 6 and don’t need a restricted diet anyway. The flappy fat under the belly near the back legs – called the primordial pouch – is not the kind of fat that you need to worry about. It is special fat that keeps the cat alive when there is no food at all (see starvation event..) and is more like a camel’s hump whose energy can only be accessed when the cat is actually starving.
The Fat Farm was designed to get cats to eat the barely palatable RC Satiety or HIlls Metabolic diets. The diets do work because they are excellent nutritionally, but not hyper-palatable, so the cat will learn to eat only what they need. After that, the diet should be able to be given ad lib and the cat won’t overeat. However, it turns out the owners could not resist giving other food and that undoes the entire nutritional advantage of using those diets. ANY different food will send the cat back to begging, and that’s hard to resist (see Kitty Blackmail pdf) so the cats who have been to the Feline Fat Farm all ended up, well, fat again.
From a welfare point of view – if your cat is mobile and fat, no problem, keep a regular check for diabetes (which is VERY treatable) and don’t stress yourself or the cat with calorie restrictions. If your cat has ever had a starvation event (locked away in a garage, got thin from a bowel problem, lived on the streets or been pregnant and not fed well), then let them carry the weight – they KNOW what hunger is and they never want to go there again.