Thanks for a helpful suggestion.
I reduced speed by half and make it move a full circle with 6400. Better 3 out of 4 tries. 4th try missed the mark by about 100 steps. Not acceptable.
Even slower missed more.
One thing appears to be true. The setSpeed does not set a constant speed. It might be what the speed is to start with (not sure), but it appears that setMaxSpeed is the real speed control. From what I can tell, the motor starts at setSpeed, increases by setAcceleration until it reaches setMaxSpeed and that is what continues.