If you increase the valve pressure (turn in the screw), you increase the valve's resistance to opening at lower vacuum levels, meaning part throttle or at idle but you also increase the occurrence of compressor surge, which is what a BOV is there to prevent in the first place.
Guys..the auzzies have use the Apexi Power FC ecu in their Altezza and have kinda solved the low torque range issue that all altezza have. Go to this link and read about it...