I just bought a Full Piper exhaust system for mine brand new for £220 delivered, Demon Tweeks had an offer on ebay, not major loud, but still sounds good when giving it some beans..
Also bought a toyo 4-2-1 Manifold off ebay for £100, not sure if it gives any gains, but it made the engine bay look better.
Didn't know the ST170 came as standard with a sports cat, saved me a few hundred quid there, as I was looking at a sports cat next.