Same car, different motors.  The VRIS points, timing, and fueling are most certainly different on the ZE than on the 03 so a chip or J-Spec ECU should really improve the curves.  There is an impressive increase in power through the entire band, but over 5500RPM, the ZE completely embarrasses the 03.  Note also that the ZE HP peaks 6000 where the 03 is 5600.  Quite possibly, this is due to different cams.  Also note that the ZE's torque doesn't peak as high compared to the 03's HP/torque figures.   It is completely possible that the US Spec VRIS points are hindering higher torque figures.