I have the 2016. Love it.
The IRS on the ‘03-’04 was an afterthought to mount on the that old platform’s chassis (it was still a Fox Body with cosmetic changes.
I have read the new one requires changing to stiffer bushings to counter wheel hop if you drag race with drag tires. The good news is that it did not break until 600 supercharged HP was applied.
Shock loading an axle by launching with a drag slick is about the most abusive thing you can do to a rear end. Straight axles in the 2005-2014 Mustang are pretty much good to go with drag tires being the only mod needed for competitive launches.
Ford upgraded that axle from a 28 to 31 spline when the 2005 came out because that was one of the mods the drag guys were doing anyway.