Can you even raise the fender by 20mm. Looking at mine, 800 not xc, I would say there's no room to raise the fender at all, unless you replace the brake lines with straight runs from the master cylinder and do away with the loop over the fender.
Has anyone checked the suspension travel to see if there's contact on full jounce?
I fitted a stebel horn under the beak and did a quick measure up and there wasn't much to play with so had to tuck the horn up inside the beak cavity.