Opening up the air intake system to make it less restrictive and increase airflow to the engine, and decreasing restrictions in the exhaust system by using larger pipes, mandrel bends in the pipes, and less restrictive mufflers, when done together, will generally increase hp and torque. By how much depends on the engine. Increases can vary from a mere 1-2 hp and 1-2 ft. lbs. of torque to 10+ hp and 10+ ft. lbs. of torque. The dyno results I've seen generally show that the larger increases are on the larger V8's and larger diesels.
Noise will result from both the intake and the exhaust modifications, and it may be a considerable noise increase. The intake and exhaust sysems are factory engineered to low noise levels, because low noise is what most consumers want, which results in restrictive systems that, when modified to increase air inflow to and outflow from the engine, will increase hp and torque.
Do NOT believe any self-serving hp and torque increase claims made by the manufacturers of exhaust systems and their components, hi-flow air cleaners such as K&N, and cold air intake kits.