What can be done to unlock more power from a 4.2 intercooled turbo diesel in a 03 GU patrol ?

Will be adding a snorkel and K&N air filter which should help a little, and I would like to keep it all leagal and not have too much bark from the exhaust. I'd also like to keep the fuel consumption down to around 15l per 100 if possible.

So what I need is a list of what should be done and in what order.