The exhaust system alone cannot increase the horsepower of an engine. The horsepower of an engine is determined by the amount of power the engine produces. The exhaust system plays a role in allowing the engine to breathe better and in turn, can improve the engine's performance by increasing horsepower and torque.
However, increasing horsepower and torque of an engine is more complex than just working on exhaust system. The engine components and the engine management system all have to work together in order to produce more power.
An exhaust system can help release restriction and back-pressure caused by the exhausted gases, allowing the engine to breathe more efficiently, which in turn can improve the combustion process. This can lead to an increase in horsepower and torque, but it would be limited.
To significantly increase horsepower and torque, other modifications would be necessary such as upgrades to the engine management system, adding forced induction like turbo or supercharger, increasing the compression ratio of the engine, and even larger engine components such as pistons and connecting rods. These upgrades allow the engine to burn more fuel, which in turn generates more power.
So in summary, an exhaust system can help improve engine performance and horsepower, but it cannot increase horsepower on its own. It is one of the parts in the puzzle of improving engine performance and power.