i don't really know the details of fork weakening. but again, when i asked this question over the summer, people said that even minute shaving of the fork steerer is not a good idea. they argue that threadless steerer tubes weren't meant to be cut into (even though threading is really not very deep). to your defense, there really shouldn't be much stress directly on the steerer tube. i was still willing to cut into my threadless steerer tube like you, except there were no shops that i could find that would do it. but if you're mr. pothole/mr. trickster, i would advise against it.