the older GT track pack bar does not have bonded bushings to the stabar like the 5.0. the bonded bushings not only add to the roll rate, they add to the front spring rate. 5.0 gt bar is 34.6 x 4.4 wall, the last gen GT track pack bar is 34mm x 5.1 wall.. very similar spring rates of the bar only, but the glued on bushings in the new 5.0 add rate.
what will added rate do? i dont know since i didn't talk with the dynamics people. the bonded bushings are a premium thing thou..
edit: the bar i was talking about was the 2007 track pack gt, not the 2007 shelby.. the shelby has the same 34.0mm OD but with a thicker 5.7 wall so it will indeed be a little stiffer than the current GT 5.0 front bar. will that extra bar cause under steer? or will it just fix body roll issues.. only one way to find out.