Very little. It is better to have duals instead of single as it is harder for the car to breath. But it can add 5-10 horse.
it varies on the manifold and engine. if it helps move air it will increase horse power, if it doesnt, torque.
It depends on your vehicle and the type of exhaust you buy.
yes it adds about 15 hp depending on the car.
3 to 8 maybey 10 but not likely