There is technically no such physiological concept as “metabolic capacity”.
The whole building the metabolism has a ceiling and it’s a lot lower of a ceiling than people these days seem to be suggesting or claiming. Metabolism does not just explode through the roof because you gradually eat more and more. It’s a nonsensical idea really especially considering the fact that the large majority of metabolic rate is determined by the organs (about 60%). Skeletal muscle for example contributes very insignificantly to metabolism (on the order of 6 cals/lb/day; incidentally, for body fat it’s about 3 cals/lb/day).
Of course we want to maximize what we can eat to be one, weight stable and two, lose body fat, but again, these figures are tremendous outliers and they do not serve as a representative example of what is possible for 99% of trainees. There are no magic tricks for any of this.
What’s being talked about with all of these build your metabolism conversations is the adaptive component of metabolism (a term used to collectively describe all the “other” metabolic activity); meaning it’s something that can change to a degree (at the same bodyweight) based on changes in metabolic efficiency as a function of change in environment; for example, eating more or less, weight training, hormonal changes, etc.
Look up spendthrift vs thrifty metabolisms. A spendthrift metabolism stores calories poorly (adaptive component is increasing and very responsive) and tend to drop body fat without much problem; think really healthy metabolism. In contrast, the thrifty metabolism stores excess cals easily and has a hard time losing weight.
There’s also a genetic component to both.
Point is, you can’t just increase the adaptive component of metabolism endlessly. Metabolism will chase caloric intake up to a point, then you get fat.