You can take the baffle out/airbox lid off and rejet for a little extra power but its not gona be anything like an aftermarket exhaust, plus it will sound like ****.
warrior 98 is right... tried it on mine and even knocked it out it had a straight through design. It had a lot of top end power but all my torque was gone. After putting on the mx-4 exhaust I could pull a gear more up hills.