May or may not help, but I never used screws to hold on my fender extender, seemed a shame to go drilling holes in decent body work. I used a load of PU adhesive and some big clamps on it whilst it set. No problems with it coming loose over the last few years, even after binning it a few times!
Maybe I got lucky on my Fazer with my bolts though, didn't have any hassle with them coming off. Though one on Mary's CBF was seized and needed drilling out.