Be careful increasing the size of the exhaust system. On a normally aspirated engine, a large exhaust system is used at higher rpm, to develop horse power. A smaller exhaust is used at lower rpm, to develop torque. And, remember: It is part of a balanced system, of intake manifold, valve train, and exhaust. If you just start futzing with one part of it you can muck it up, and actually cause severe loss of both torque and horsepower.