Replaced my plugs at 15K miles with none of them breaking off. I followed the Ford TSB on the subject. You shouldn't "need" new plugs for 100K miles (the one good thing about the factory platinum plugs), but if you anticipate needing to change them (i.e. blowers, turbos, nitrous, etc.) then do it sooner than later and go with a good copper plug which has better thermal conductive properties than platinum. Colder heat range would be great for power adders, also. I used FRPP 3V0 copper plugs that are one heat range colder than stock and they made a world of difference in how my supercharged '07 GT runs!