Good, I'm glad you came to this conclusion. I understand the frustration of this kind of thing and how easy it can be to write it off and think it's ok, or think there is a manufacturing defect and just be mad about it (I'm not hating, I have been there myself more times than I care to have been there). You, your mustang, and all of your fellow drivers around you, will all be better off.You're all correct, of course. After mulling it over I remembered hitting something hard a while back--don't recall if it was the RF, but obviously it was--so a new tire is in order.
Thanks all for bringing me to my senses; with the suspension upgrade I'm ready to push it a little more.
Edit: Fortunately, the wheel wasn't damaged. The black stuff wiped off easily; it appeared to have been rubbed-off the sidewall. Looks like I hit it as hard as I could without damaging the rim (still trying to remember how/when I hit it).