Of course they will catch. I had ET32 offset Advan RG's on my Glanza, 15x7" and even though the rear arches were rolled, they were grinding like mad. ET20 will be sticking out an extra 12mm, they will rub a LOT more. You can get away with the fronts by using wider fenders, I have ET10 offset Rotas on my Glanza fitted with my fenders. They won't rub a lot, but the rears will be going mental. I'm planning on having the arches flared and will probably set the coilovers to the hardest setting to it's less likely so bounce much and catch the arch.