It's probably like Teutonaddict said. You loose low end torque with less back pressure. Next time you pull up behind a 330, look at the inside exhaust pipe. You'll se a little butterly valve in there that will close when someone pulls away from the stoplight. It'll open again later on for increased airflow. It's to help with low end torque.

Maybe install the glass pakcs or X-pipe? It's a catch-22 since if you increase back pressure too much, you'll lose power up top.