Madz

OK, if it is genuine turbo charger do not touch the adjustment, it is set at factory (Garret0 and can't be reliably set afterwards. I know some people have fiddled with it, my recomendation is don't.
It would be interesting to  note EGTs when under load whilst lacking power as that can point to a problem. If the temps are low that means not enough fuel and if very high means less air however trick is to understand that relationship. 
15 psi boost limit is standard factory setup without Dawes valve so what is the point?  So at what RPM are you seeing the 15 psi boost? 
ZD30 will usually operate around 8-12 psi happilly without you noticing "huge" lack of power you describe. Indeed as strange as it sounds at lower boost around 12 psi the ZD30 seems to have best fuel consumption vs power output but it will start to struggle if pushed harder without extra air comming in which will be indicative by rapid increase in EGTs once loaded easily flying past 600 C (not good). 
As an example I have mine setup with Dual Dawes/Tillix valves and manual overide where I can limit boost to max 14 psi vs top 22 psi in auto/max mode
That will see the same hill climb at 500C + /14psi and slowing vs 380C/19 psi and accelerating. Mind you mine is heavily modified including customized ECU remap.
So let me get this straight:
You have Dawes valve installed as boost limiter?
What controls spool up rate? is it factory ECU vnt solenoid?
I think you could have badly set up boost control but I still don't understand how you can even reach 15 psi then.
If you could list/obtain boost psi vs RPM vs EGTs and Go pedal position vs Air flow meter voltages that would help to pin point likely causes. Thats what I would do in order to narrow down the issues. A laptop with ECUTalk software and USB cable conected to diagnostic interface would help see and log the relevant data. 
Generally speaking Nissan Patrols with ZD30 4 cylinder engine from factory are pretty low on power anyway and do struggle of the line but once above 2000-2500 RPM they improve a bit showing most power around 2800-3000 RPM.  Still nothing to crow about though!
That engine struggles with the design choices Nissan made in lame attempt to control emissions and that pathetic fuel injection control from Bosch makes the things even worse. 
Regards