Engine performance is increased with a better exhaust system. It's all about efficiency with engines, so the quicker an exhaust can get rid of exhaust fumes from the cylinder, the better.

The ST feels ever so slightly more responsive with a K&N 57i induction kit at about 4000 revs (form my own personal experience). The increased airflow into the engine helps quite a bit I think. At the moment, however, I still have the standard exhaust system. I'm waiting until the warranty runs out in December until I buy a new exhaust. Together, the new exhaust and the K&N will be a great combination.