Easiest upgrade is the 11.75 factory rotors. With the correct brackets, you can retain your original calipers which makes it a very economical upgrade.
Next size up is 12.12 rotors, but these are custom units like those used in Nascar. You could do these with Wilwood parts, but there are no off the shelf kits and you would need to piece all the parts together to do it. This would be a hub assembly, rotor hat, and rotor.
That is the biggest you can go with 15" wheels.
After the 11.75 rotors, next upgrade would be light weight calipers. The whole point of this is not that they create more braking force, but they reduce unsprung weight which allows the wheels to react faster. Easiest choices here are Viper or Wilwood calipers as there a kits out there readily available to bolt them on. I've heard that there may be some companies making adapter brackets for Mustang Cobra calipers and Team 3 Bakes in Australia is developing kits for adapting C4-C6 Vette calipers to mopar spindles.
Caliper comparison photo for your enjoyment.