PD 130 uses a Garrett VNT15 turbo. Boost is controlled by vanes in the exhaust side of the turbo, facing the turbine. When the vanes change direction, by what looks like a wastegate actuator, the speed of the turbo is controlled, which in turn control's boost pressure.
The N75 valve is basically a boost control valve. The different type's control at a different speed, the lower the letter the faster it opens/close's. The slower it operates the boost can be held for longer. If the boost is held for to long, the ECU can go into limp mode and restrict performance.
Mapping/chipping controls the signal to the N75 valve, but takes into account the amount of fuel needed to keep the correct air/fuel ratio. If the valve is increased to a higher letter, this alter's the correct air fuel ratio, and will run slightly leaner. If one goes too far and runs alot leaner, combustion tempretures will rise, which is not good for the engine or turbocharger.
Some increased boost can be gotten from this valve, even after it is chipped. If the car has been mapped on a rolling road correctly, I would hesitate to change the valve.
This is only my 2 cent worth on the N75 valve, and the differing views will be argued over for a long time.