I recently purchased the MU-2, what a great airplane, too bad I didn't do it when flying XP 10.
However, I too experience the high power when taxiing. Somewhat solved it by using procedure from another poster in this forum - put the blue condition levers (CH Throttle Quadrant) just forward of Taxi, then the Power Lever will go back into Ground Idle and Reverse regions. I can then adjust the Power Lever so as to go in and out of Reverse and can somewhat control the taxi speed. I'm running X-Plane 11.05. Hope this helps the troubleshooting.