The current gen Mach 1 has a very nice set of upgrades when compared to a standard Mustang GT. The torsen diff is wonderful, better subframe and a bunch of other goodies in addition to the better brakes.
I test drove one around the block, and quite simply, it felt the same to me. In fact, the GT350 and standard Mustang GT feel more similar than different. Despite the wildly different specs. So in that light, I'm not sure it is a big step up in "fun factor".
However, all those goodies will pay off on a track day, no question about it. Or, of course, if you drive like me....