Since we're kind of talking about everything here, these are some shots of a 2015 test mule. If you look closely you can tell it obviously is running IRS. Look at the wheel nut and then on the rear shots you can see the spring cup and some other bits.
For background, the S197 chassis was originally designed to accommodate IRS. The first GT500 was supposed to get it, but for one reason or another it didn't work out. Ford designed a bolt-in test system for it that you can see here, where it was found in an old warehouse:
DrivingEnthusiast Blog | All about the missing S197 Mustang IRS
Obviously the one in testing now is quite a bit different as you can tell from the pic.
I also notice in these pics some nice front calipers and the rear rotors are definitely bigger diameter. If everything comes together as we expect (~3300lb curb weight, 450hp) the new car will be even more of a beast than current. The only question is how will it look.