There are 5 normal ways to increase the boost on most turbocharged cars:-
1. Weld the wastegate shut (not recommended)
2. Adjust ECU programming (not always possible)
3. Fit an electronic boost control computer (recommended)
4. Fit a mechanical boost controller (like a dawes device) (not heard of one on a skyline, hence not recommended).
5. fiddle with a restrictor....
... many turbo cars have a restrictor in the pipe that runs to the boost control solenoid. Eg. 200SXes. Removing this or making hole larger can increase boost. Tends to make the boost a bit out of control (spikes etc). It can make your car hit full boost at part throttle. Hence bad idea.