Horsepower is an arbitrary number when it comes to a car that has only modest modifications. Power to weight ratio and torque is what you are looking for. To say I'm going to add a K&N air filter (Advertised at 5-7 horsepower typically) and add headers (typically 10 horsepower) is in a best case scenario where the air flow is maximized from the intake throughout the exhaust port of the heads through an unrestricted exhaust, and a high flow catback with high flow mufflers. If any aspect of the system is not structured to maximize air flow you will not achieve these generic numbers you are looking for. Increasing torque and removing unneeded weight will help in improving acceleration. But to know for sure you have to dyno the motor. If the motor is in the vehicle you'll get 2 results, one will be HP genereated by the motor at the flywheel and an estimated amount of HP at the wheels, which will take into account loss of power through friction