Well, a STB will do a couple of things, It firms up the whole car, it adds rigidity to the shell, reducing flexing when you're driving hard, helping the suspension do its job. The cool thing is, you do not HAVE to drive hard to notice it, your first tight turn you make you will notice it even more, SUV's notice the change more than small cars because there is so much body flex.
And yes, with the GTSPEC, no other mods, just bolt it on and go, take the nuts off the top of the struts, put the bar on, torque them back down, and go enjoy it.
Hope that answers ur question