teamgeorge,
Turbo in CRDi is controlled by a range of sensors that feed info to the ECU, so we have been led to believe. I assume you have read the threads about reprogramming the ECU (Martin from DW Tuning). From what I have gathered, reprogramming the ECU, or using a box like Shambles has installed, is the best way to change the boost / fuel delivery and to gain more power.
I have fitted a Scanguage and can see the MAP (Manifold Air Pressure) as I drive. MAP can be as high as 30+ psi when it needs to be (just plant your foot and the respose is evident), but it sits as low as 14.8 psi when idling. I have also noticed that a 2-3 psi gain is evident when the aircon is operating. This small boost in the MAP compensates for the A/C operating. There may also be a change in fuel pressure / delivery tha I am not aware of.
Now, the turbo is variable vain and is, I assume, controlled by the ECU and the range of sensors, including the throttle. So, what I gather is that the ECU delivers the precise amount of boost and the correct fuel ratio for the job at hand. Messing with this balance is not something I would want to play with.
I reckon the best results would be had by either using an after market chip or a DW Tuning type reprogramming.
Just my thoughts. Others may have differing opinions or variations on the theme.
Bob