exhaust system, air filter, boost controller, re-map the list goes on,
to be honest m8 it down to u on what u have done and what sort of performance u lookin for
a de-cat will make a differance to performance and give it abit more of a grunt,
as i said its up to u.
on my e4 i'm runnin a full 3" system from turbo back with de-cat hks mushroom filter e6 injectors(560's) uprated 255 fuelpump e6 pistons and gettin a e5 ecu this weekend and a re-map soon,
also goin for boost controller, uprated 272cams adjustable pullies and a bigger turbo for a 400/400 set up (hopfully)