Mustangs can handle quite well when setup properly. One of MM's basic suspension packages was tested in 93 on an lx fox by road and track (or some other major car mag) and it ran the 3rd fastest slalom time they ever tested. Granted that number has dropped in the last 15 yrs. but thats still pretty impressive. A panhard bar makes a huge difference on the rear end. I have no problems pulling away from STI's at the road course I go to, in the dry. In the wet I can keep up with them on the corners but I have troubles putting power down on the straights so they tend to pass me there after a few laps. But thats subjective, I have more hp and theres also drivers and tire dofferences to consider. Now if I someone would let me drive their STi, then I could give you lap times to quantify.
But hey, I'm gonna do some research and maybe I may end up getting a rear sway bar too.
Oh, and 95 cobra guy, what battery box is that and how do u have it bolted on/down. Do you have to get underneat the car to unbolt stuff to open the box and get the battery out? I need a nice clean setup like yours.