I would prefer IRS in my 2013, but it is what it is. Ford got the solid axle setup as perfect as practical. The IRS competes in most categories and yes, the 2015 would be significantly lighter with a solid axle. However, only the 5.0 has really gained weight. And the weight is added to where it gives the most advantage. As for drag racing, FRPP has already came out with kits that replace the frame bushings with aluminum units, and new HD half shafts are available. With those, the Mustang has been rated at well over 600 hp and no failures. In a few years, no one will think about a solid axle in a car. They may even switch to IRS for trucks.
2013 MCA Edition-Sterling Grey
Roush body kit - Splitters/hood scoop
Whiteline suspension w/Koni shocks