The Bullitt came with a 3:73 limited slip rear end...essentially the same rear end as the GT500 of the same year. Lower rear end ratio than the GT. Different cams than the GT, giving it another 500 revs before redline. An adaptive tune, lets you run regular, or get a power boost with premium. 2500 on the tach when cruising at 80.
Cindy loves the seats for comfort. She complained of the sports seats in the old '72 911S giving her a backache. The Bullitt seating is leather, also the same seats as the GT500.
So, the Bullitt is "basically" a GT, but it has enough added goodies to make it unique from a GT beyond it's sleeper looks.
As I said on another thread, it's probably my last car. I was sorely tempted by the new GT350, but for the driving I do these days, the Bullitt does all I need. That, combined with Ford dealers asking 5 figures additional markup helped me decide I didn't need one.
Of course, no Mustang could be considered as cool or as fast as a well set up 944.