I currently own a AU series 1 and want to fit a complete exhaust system have been looking at a JMM DEV1 kit which is about $1300 fitted and uses a modified cat? Does anyone know what they do to a standard cat and can i do better for the same amount of money.

Unless your doing forced induction or your existing cat is rooted, dont worry about changing it.

You should be able to get a 2.5" made up and some Pacemaker 4499 fitted at any exhaust shop for around the 800 mark.

CHange your o2 sensor when you change exhaust. It's probably due.

they use the same cat but all they do is open up the inlet/outlet holes a little (to around 2.5").

just means better/more flow.