Metabolism control seems to be based of medium term intake rather than fat stores. Either that or there is a set point which is quite hard to move. Or both. Handling the fat does increase basal metabolic rate to an extent. But only so far. The abovementioned slowdown starts with mild thyroid suppression and shows up in bloodwork. This can cut BMR by up to half without starvation. (25% calorie cut from high levels is nowhere near enough)
The problem is, this reduces will to both diet and exercise and induces a low grade depression.
I only have my own metabolic panels taken at my expense weekly during a vegan diet (vetted by a dietician) attempt. A failed one to boot. From correct baseline to borderline hypothyroidism in a matter of weeks. Quick rebound when the attempt was stopped.
The study would have to be careful to exclude people with known preexisting conditions that cause obesity and only take in "idiopathic" subjects.
The most interesting related study I found is the Right Track long-term study of adolescent health related behaviour and total factors. It is currently in progress. Hopefully it won't disappear.
To be at that level of starvation would require the person's body fat percentage to drop to essential levels. "Starvation mode" in normal weight and overweight people is a pernicious myth.