Silly me, Saw that the $8.50 HiLetgo 9-axis IMU / MPU-9250 lists VCC of 5v and didn’t think they would run the I2C at 3.3v.
I made a nice adapter cable, plugged it into the GoPiGo3 hardware I2C jack, affixed the board nicely to a “first generation” DI / Grove sensor mount that only cost 50 cents, fired up a read of the “WHO AM I?” Register, and saw the expected 0x71 “MPU9250” response, and proceeded to attempt calibration.
Only when the output values seemed to be jumping about wildly did I begin to suspect something was amiss.
I connected the board direct to another RPi and discovered much better behaving output values.
Guess I should have done more investigation before getting excited about saving some money. I don’t want to wire it direct to the GoPiGo3 GPIO connector so the final result is to toss this board and buy the DI IMU.