Dont care about the whole torque arm argument you are thinking of, hell a show just switched a Camaro to a Mustang style rear suspension and raved about its level of improvement. IRS is cheep to slap in (how much for a F-body?) and the 4-link is easy to make very track worthy.
Sorry to ramble, pain meds just kicked in, chemo is a biotch
Yet a majority of the road racing Mustangs are swapping to the Torque Arm/ PHB(or watts link) setup on the rear of their mustangs. Not to mention going to the similar SLA front suspension as well. A buddy of mine is a dyno tuner at our local Mustang race shop that primarily deals with road race cars and they don't use the IRS setups(while a good setup) as much.