I've got a diesel power module http://www.kpd4x4.com/nissan-navara-dual-cab-st-x-may-2010-diesel-power-module-2-5-dci/
Noticable difference in power and torque (26kW supposedly), no other mods, only had it a few weeks but money well spent so far.
Easy install and uninstall if needed. My cousin...