I think the issue is more that the overspeed downhill (at 5-6% grade as has been posted here recently) places rather extreme impact stresses, and at 16 the body is still developing and maybe not ready to handle the higher stress loads.
It does appear that resisted or overspeed by itself does not seem to have much of a performance improvement (that was posted recently too and there other papers to that effect), so it may indeed be that the overspeed IS needed to get the gains that have been mentioned. I’m going to do this stuff for a couple of workouts with a high school sprinter, but he’ll be 18, not 16 at the time, which is a big difference.
Maybe you can run on the flat with the wind and not go to the full overspeed downhill?