Could it be because our motor doesn't have a MAF sensor, it use's a MAP sensor.
That with small increased in flow, the ECU doesn't adjust the fuel perfectly.
Making the car run different, and not at peak performance.
Map sensor aren't good with increased flow corrections compared to MAF.
Increased flow would not be detected by the MAP sensor.
Since it mesures pressure/vaccum changes, and increased flow doesn't affect pressure or vaccum.
On cars with MAP sensors, normally any changes in intake or exhaust requires a retune to optain peak performance.