If you dont plan on changing the turbo but want to maximise your HP follow this setup:
1. 3" dump+front pipe off the turbo to the cat. (this is help the stock turbo to spool 500rpm earlier)
2. 3.5" cat back exhaust. If max hp is your aim go for something like an apexi n1 system which has no resonator's and is pretty free flowing, downside is its quite loud and you could get a defect.
3. Stronger/heavier clutch, just a single plate unit with 2200-2500pound pressure plate and a lightweight (ogura flywheel is 4.5kg's lightest around) flywheel. This will help you put the power to the ground and make the car more free reving.
4. Boost controller. If you are doing this on a budget a simple bleed valve will do the job or if you have the dosh buy an electronic one as they hold boost better, dont spike and build it faster usually. However if you decide to get an aftermarket ecu some of them have boost control built in.
5. Aftermarket ecu or SAFC. If you want to make maximum hp go for the ecu it will cost more but will make the car much more fun to drive. A wolf3d has built in boost control as well which is a plus but more people around australia can tune the powerfc.
6. Exhaust cam gear + better brake pads. Cam gear will give more midrange and you will now need some better brake pads to help you pull up.
With all this you should have somewhere between 280-300hp (dyno dependant) but this is what we have been seeing in perth on a dyno dynamics accredited dyno.
Have I left anything out?