with the advantages of lower weight and lower cost(i swear, if someone references that bullshit motortrend editorial...).
I think when you build a vehicle with an IRS in mind it comes in lighter than a solid axle and probably by a good margin. I think this weight advantage myth comes from the previous gen mustang application requiring a bolt-in solution.
Take a look at any sports car rear suspension. It surely has to be lighter than that gigantic steel SRA under the mustang.