RE: New Hanger 9 p-51 mustang **Trainer?!?**
I didn't actually "remove" as indicated in the instructions here, it on mine, rather I purchased a replacement mount, so that I could re-install the flywheel if I wanted.
After pulling off the flywheel assembly and replacing it, I've have had no idling problems, although just to be safe I set the idle speed up a tiny bit.
Removing the fly wheel had a slight effect on the left yaw at takeoff... improving things a little bit.
I still have not figured out if it is worth removing though.