Yes that is what I said the 3 bar sensor under reads by 20%, so effectively the ECU will be trying to provide 20% more boost than it is already.
A typical phase 1 map may increase boost by 5-10% max. A 20% increase is big.
To increase boost the ECU will keep the VNT vanes closed up more, this increases back pressure in the exhaust a lot and impacts fuel economy, the turbine will be spinning a lot faster than it needs to and you are likely to get limp mode due to underboost or possible CEL for out of range as the sensor will be reporting a vacuum at idle and low speed which is never the case in a diesel.
Typically a 20% overboost will cause the car to go into limp mode to protect the turbo.
Better to change the sensor over at the point where it's mapped. Or have the absolute minimum amount of running on the incorrect sensor by changing it shortly before it is mapped.