Im running 17x8 ET35 rims, coilovers, and a bespoke rear disc set up which sits out further than any other rear disc set up I've seen (and more so than the old disc set up it replaced) and my arches only need very minimal trimming when I get round to it. 2-3mm maybe? It shouldn't be that hard. Also I would cut the arches rather than roll and fold them, unless you are welding them up, as thats a perfect environment for rot.