I have a 97 - 3/4ton 12 valve CTD auto trans. It's a basic "stock" truck. The only thing that I've added is a trans. temp. gauge and a K&N air filter. I talked to someone who said it's great to get more air in but you have to get that air out too. He suggested I install a 4" exhaust system. That's a $500 bill! Would I notice a performance increase if I were to do this? Does this increase the HP, or better mileage? What is the benefit of doing this?