From what I always read/heard about the Cobra owners is that they didn't like the IRS in drag-racing applications, and many would swap them out for the solid axle. The IRS was weak, broke easily and caused awful wheel-hop. However, wasn't the Cobra the only one to have IRS? The rest still used solid-axle? That's the problem, you have a platform (the Fox) that was designed for and always used solid axles. Try to add an IRS and its just not gonna work, and it didn't.
Ford obviously had the ability to make a plenty strong and capable IRS, see the GT. And here they had the opportunity to really make a difference with the Mustang. Design a whole new car and still use old, less-capable solid axle? Just a dropped ball if you ask me. That just kinda irks me. A chance to reinvent an automotive icon, and corners were cut.