The differential pressure at 8 m/sec is roughly 32 Pascals, at 2 m/sec it is about 2 Pascals, so the maximum effect of your observed sensitivity will be on the order of 0.5 m/sec near stall speed and 0.2 m/sec at cruise speed.
That’s assuming my calculations are correct. The relevant code is here: https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Airspeed/AP_Airspeed.cpp#L266