An ECU isn’t a product that makes power; it’s a tuning tool. On an unmodified engine, it will only make power that was “left on the table” by the manufacturer.
However on a turbocharged engine, an ECU will allow you to increase the boost while being able to control the tune. One bottleneck when turning up the boost is the injector size; fitting an ECU allows you to change the injectors and be able to correct the tune. It also removes the bottleneck of the factory airflow meter by converting to a MAP sensor based tune (also called “speed density”).
The additional power that you can make depends on other supporting modifications such as stronger rotating assembly, freer flowing exhaust and intake systems and so on.