Horsepower is a direct function of airflow. Simply put, if you can’t flow the requisite oxygen, then you’re not going to meet your horsepower goal. With that said, the factory mass air meter, like any other sensing device, was only designed to register so much airlfow. After a certain point it will no longer indicate that the engine is ingesting more air, and the engine will at best fuel cut, or at worst run dangerously lean. The fix for this problem is to install a larger MAF housing, along with a new transfer function for the sensor.