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.
Yes, on my XC w/ABS I was able to raise it 20MM safely without stress on the brake lines. Anything more on my bike may have caused problems. I had a dealer in Reno, Nevada claim they raised one 40MM without running new brake lines. There was no way I could go that much on my bike. The front fender setup on the roadie is different so I don't know about those.