I can't recall the specific number, but something like 72% of braking is done with the front tires on a Mustang. So if you have an equally bite-y pad in the rear, you risk locking up the rear/getting squirrelly under braking.
Cobalt pads and the similar types for GLOC, Pagid, Hawk, etc. are true race pads - so they aren't prioritizing street manners (cold bite, noise, dust, etc.). They're just a lot less accommodating than something like the stock (which is the best pad I've experienced for combined use).
If I were spec'ing pads for your car, I'd go XR3 front and XR4 rear. You can probably get away with a higher temp pad, but you'll likely just be overloading the front tires.
