Make some tests with loiter, moderate and high speed, and the same with STAB and the “dreaded” guided modes (but this latter is just a fancy Loiter mode) and post the logs. It will show if the baro is really influenced by the speed or not, or if the barometer is more heavily over-balanced in the IMU.
From the Methodic Config:
"
10.2 Baro Compensation flight(s)
Follow ArduCopter’s baro compensation Wiki and/or use the Lua script provided by Yuri in the forum.
Use ArduPilot Methodic Configurator to edit and upload the 41_barometer_compensation.param file to the FC.
Now do the flight to collect the data and analyze the logs to see if the barometer is correctly compensated and insensitive to wind."