IRS has it's advantages and disadvantages over the solid axle setup. IRS may weight more as someone has mentioned before, but is has very little unsprung weight, unlike solid axle which is all unsprung weight, so that's better for handling already. Extra weight in the back is probably a good thing for a mustang anyway, because 5.0 cars start off with 58/42 weight distribution and I'm sure mod cars are not that far off as well. Adding weight to the car affects the handling in the bad way most of the times, but better weight distribution will offset the difference, and you can compensate for more weight with stickier or wider tire all around.
Disadvantages: IRS suffers from wheel hop under hard acceleration. Solution lies in removing all the compliance from the cradle, which takes away from the ride and comfort that people are after most of the times they swap in IRS. I've also being told, and read in a few places that because of the geometry constrictions, IRS will not deliver as much traction as a properly setup Torque Arm or 3 link car would.
