Follow up report on the Pro-Spec brake disc. Checked the disc and surrounding plastic areas for wear, none found. As stated before I slightly rotated the carb to provide a little more clearance and after quite a lot of running there were no marks on the back of the carb. Verdict, it fits fine unmodified with my N12LL3 motor with the slight carb rotation. Brake force was not particularly any greater, but I suspect that the bigger brake disc will provide less overheating. It added quite a lot of rotating mass, 13g vs 6g for the old disc.