Alloys with any "dish" or lip are a nightmare for fitting brakes, makes more difference than offset in many cases! Or buy a kit that fits in the first place, the WMS 277x25 kit should fit under most 15's, and failing that there's another 3mm of spoke clearance available with the 275x22 size.... ;) The trouble with changing the wheel offset from factory standard is it messes up the handling, tends to induce torque-steer and squirming under braking.