99% of Mustang buyers can't tell you the difference between a IRS and a SRA, of the 1% that can 99% of those can't tell the difference driving on the street. The only advantage IRS gives the user is that .05% difference it takes to win vs. a SRA on the track! The IRS on the S550 platform will be replaced with a SRA within two years or the whole platform will be gone in three. It cost twice as much as a SRA and gives little performance value to the vast majority of Mustang buyers (may be even more expensive then that). Ford put it in to compete in the European market. That market doesn't buy American Muscle cars. The Camaro with a IRS has been available in the European market for 5 years and they are lucky to sell 1,500 a year. Most of the Mustangs and Camaros sold in the EU are bought by Americans!
The IRS in the S550 is a failure, it needs getting rid of now, not tomorrow!
Odd, because most of my friends (some who aren't car people and some that are) noted how unplanted the rear end of the car seemed under any sort of power through less than perfect roads. They wouldn't be in the market for a sports/muscle car anyway but they still noticed. I believe Ford left a lot of room in the S550 IRS as they did with the 03-04 Cobras. They probably missed where it should be in terms of NVH for the enthusiast, but to people with the V6, EB, and a few 5.0s the ride is perfect and they probably will never see wheel hop.
Performance pack should really include better bushings/subframes/halfshafts or whatever it takes to eliminate most or all wheelhop. Most people in this segment won't care about a little NVH considering many are going to upgrade exhausts, lower the cars etc.
I went from my 11 Shelby to an 06 Viper and even though its a different type of car, the IRS in the viper is amazing to drive on the streets compared to a SRA car. I have very high hopes that the GT350 takes on the IRS issues and makes a hell of a car.