[ZOERA-SC7XX]

For street use, I'd leave the rear sway bar off. What it does is keep more weight on the inside rear wheel with high-speed tight cornering (almost the same effect as higher rated rear springs), preventing weight transfer and making the rear end loose, something you don't want in a Cobra. It's great with soft rear springs and smooth roads, or on a high-speed race track with race tires, but may not be too desireable with standard or stiff rear springs on normal roads. Remember those old films with Ken Miles driving the Shelby Cobras on the track with lots of body roll? He didn't use a rear sway bar. He let the chassis do its job.
With that said, every Cobra is different and every driver has their own preferences, and with all the different chassis setups available, there's no hard rule, but just because you have a rear sway bar doesn't mean you'll have a better handling Cobra. Less is more.