Its not that hard to belive;
The '05's front
suspension is a coil over strut very similar to the strut front
suspension that BMW 3 series uses, including the M3. So are you suggesting that we start arguing that an M3 can't possibly out handle a 350Z because it has struts?
The '05 GT has 53/47% front / rear weight balance verses the Cobra that has 57 / 43 F/R weight bias. Ever wonder why those dinky under-powered Miatas are so fast on an Auto-X course?
The '05's unibody is 50% stiffer in twisting and 30% stiffer in bending. The unibody is stiffer than a BMW 3 Series. Unibody stiffness does wonders for handling as it lets the
suspension do its job w/o having to compensate for unwanted structure distortion.
IRS has more to do with comfort and handling on bumpy roads. On smooth roads a good live axle setup like the 05 has will be as good if not better than a kludge IRS like the Cobra has, which was a modified version of the MN-12 IRS adapted to fit into the old Mustang. The Cobra IRS is a long way from being an optimum design.
As far as the tires go, it really doesn't matter what you put on a car that has a flexy unibody and bad
suspension design, fat sticky tires will not make up for all the other short comings of that car.
The most important thing is that the '05 is a clean sheet design that was
specifically designed to be a sporty - performance car. ALL prior Mustangs were based off low cost economy cars (Falcon, Pinto, Fairmont). The last Mustang started out as a 1978 Fairmont that was an out-dated design the day it was introduced). That same basic Fox platform design has been band-aided for the last 25 years.