Have you installed a catch can and EGR block?
These are usually what people do first, to keep the engine clean.
Some say the EGT block gives a small performance boost as the turbo spools quicker - unsure about that, i don't have first hand experience.
Then moving onto the standard turbo car modifications, exhaust and intake restrictions.
Larger exhaust system, usually 3inch from turbo back will do the trick. Dont get stuck up in the exhaust size argument, its a turbo, turbos dont like back pressure, so the larger/free-flowing the exhaust the happier it will make the turbo.
Most Navs just get snorkels to feed cold air tot he airbox, useful modification also for those who go offroad.
If your going a bit silly you might want to upgrade the airbox to something with better flow characteristic.
Larger inter-cooler or W2A inter-cooler setup, something batter than factory to cool the Air will net additional torque + hard inter-cooler pipes that don't balloon out.
now if you get this far, id look at chips to change the fueling of the car - this will provide a big difference however i like to think its better to do the above before adjusting the fuel. you could get a chip straight up if you wanted too.
i guess if you do all that,you could up the boost on the nav, or replace turbo.
