However, I am not sure there is no gain to be had from a change in digestive bacteria in terms of caloric intake, because chemical changes in the digestive process could conceivably alter the efficiency of energy extraction.
In the case study linked above, the woman who received fecal transplant and gained weight was placed on a medically supervised liquid diet and exercise program, and still kept gaining. She had never been obese prior, so it's not as likely that she had the kind of dietary habits that would predispose her to obesity. Also strange: her weight gain closely tracked the weight gain experienced over the same time by the fecal donor, despite dietary and exercise differences between the two. It was enough of an effect that the researchers who presented the case study have since ensured they only use fecal donations from healthy-weight donors.
That all being said, I do think you're right that psychology of obesity is a very important aspect to consider, especially that of food addiction. I hope this is solved in our lifetimes, because it will be interesting to discover just how much gut influences brain and vice versa.