A lot of people are misinformed about the SN95 chassis.
The 1994-1998 Saleen S351 mustangs were tested as handling better than the comparable year Porsche 911 as the Saleens were equipped with conventional stock style lowering springs, racecraft shocks, and stock 4 link with a panhard bar. The Saleen S351's were very successful on the track, and beat a good number of Porsche 911's.
The stock 1995 Cobra R Mustang with conventional eibach lowering springs, and Koni Yellow shocks with stock 4 link, no panhard bar pulled a .96 g during road tests on stock tires. That's almost just as good as a 2018 S550 GT mustang that pulls .98 g on the skid pad. The 1995 Cobra R is lighter than the S550, and the 1995 Saleen S351 is even lighter than the Cobra R.
I'm certain my 1995 GT pulls more than 1 g in the corners. My car has Eibach prokit springs, Tokico blues, MM rear lower control arms, MM subframe connectors, Upper/Lower MM front Chassis braces, and Polyurethane swaybar bushings/end links... I'm also pretty sure my T1 wing pushes the ass end down quite a bit. My Nitto 555 G2 tires also made a huge difference. The rear of my car does not break loose in corners, or get sideways like it did when it was on stock suspension. Modern tires do wonders for these cars. My car also puts down 357 lbs of torque to the rear wheels.
With that said the IRS is a great swap for people who want a less bumpy ride, or a better handling ride at the limit, but only if you upgrade the bushings. I'd think that if you were tracking the car an IRS with upgraded bushings would be Ideal. However I know the solid axle Griggs GR40 96 Cobra is quite possibly one of the best handling cars in the world. So I don't know, it's all up to preference.