I want to mention that based on the limited photos of the 2015 mule IRS, it is obvious that the rear springs AND the shocks seem to be mounting in the exact same coordinate where the solid axle had them mounted.
It is becoming evident to me, that they might be building 2015 mustangs with IRS and SOLID axle mustangs on the same assembly line.
To me this is very possible since chances are the IRS won't be advisable for 650hp coming from a GT500.
Also, the rumor is the chassis is STILL going to be the S197 chassis they have been using right now, but just with upgrades. (this explains why the 2013 mustang body seems to fit easily with no issues on the mule they are driving around.)
To me it makes sense to have the option for a SOLID axle. I am even wondering if they plan to put a solid axle in the 4 cylinder and 6 cylinder mustangs.
And I would assume GT500 owners going to a drag strip want the solid axle.
http://www.drivingenthusiast.net/sec-blog/?p=13173