Do you know the AR ration of this Saab Turbo? Making sure it is sized right is a key to your powerband question...
The smaller the AR the quicker it will spool to boost, small AR = power down low, a larger AR = power at higher RPM... That is a very simplified version, there is more to it than that of course...
The A/R in a relationship that is obtained when dividing the interior area of the turbine where the inner walls are found, through the turbine housing radio from the center to the tongue as the illustration indicates.
A/R values are expressed as .35, .47, .68, .84, 1.00, 1.15, etc.
A small A/R indicates a small interior volume in the small turbine and a large A/R indicates a greater volume.
At a minimum A/R the motor's response is produced at small revolutions per minute but at high revolutions we will not achieve an adequate caudal. We should always find a compromise between achieving the lowest response possible and have enough caudal at high revolutions. The picture below is for reference: