If you want a simple, quick answer, just ask yourself if any super or hypercar has ever had a SRA in the back. The kinematics (theory) of the IRS alone has it over the SRA in spades.
Now as it relates to SN95, just keep in mind that Ford shoehorned an IRS subframe into a car that had no intentions of ever having one. And while they did an excellent job of achieving that, it still is not the best IRS as a system in itself. Really the tradeoff comes in the form of increased weight because the IRS subframe for SN95 needs to be very beefy due to the mount point locations being so far apart (and because of this the control arm geometry is not the best either).
The bottom line is that a solid rear axle with some precision components will get you into drive-fast-but-confidently territory in well under a grand spent. It will transform the Mustang and you'll want to drive around like Mario Andretti all the time with the newfound confidence. But it'll still kinda ride like a lumber truck and wear you down after a while. But even with just minor upgrades to an IRS retrofit (things like better bushings, springs/shocks, etc) the IRS will always edge out the SRA car in every category except abuse-taking-potential at the drag strip. It all comes down to that.
My advice to everyone I talk to on this subject (having gone down both rabbit holes during the course of a decade) is to modify what you already have as mentioned above. Wade into the ocean, don't go for the deep sea dive right away. The SRA modified to a substantial degree can give you benefits of handling in the curves while still being built for drags and spending little money while doing it.
Me, I have no interest in drags anymore so I went full retard with the IRS.