[BigAls87Z28]

Its evolution of cars. IRS offers much better poise and agility, not to mention helps on packaging a vehicle.
IRS isnt just for track, but for ride, comfort, and steering and control ususaly feels better with IRS. You also increase the envelope on what the car can do.
The Mustang GT with track package has shown what you can do to maximized a solid axle set up, but it suffers tons over bumps in the road. The Camaro and Challenger have much better ride as well as better performance. While the Mustang's steering is better, the Camaro SS tends to get better reviews as far as its poise on the track, vs the Mustang.
Mustang will deffinatly turn up the pony car wars, something we havent seen since 87 when the fbodies saw the return of the 350.