it doesn't mount to the ferders. let's put it this way...we took the stone guard off, moved the bumper mounting brackets back to the next set of holes, notched the stone guard and replaced it...about 2 hrs. work and that was because we were working with new paint and was extra careful not to scratch it. it looked like it was designed to be tucked...
the bumper is mounted to a shell. the shell is mounted to the bumper brackets and the stone guard is mounted to the shell.
We moved the bumper bracket to the other set of holes back 2.5" . We cut 2.5" off the front of the stone guard and it worked well. We considered notching as 74 gold showed, but this route seemed a lot easier.
I pondered going your route for the reasons you mentioned for quite some time. My concerns were getting a good cut in the stone guard. I agree your method would end up with a more finished look.