Did you even read the other thread at all? This was already discussed.
Yes there is unused rotor space with this kit. Just like on stock Cobra's. Just like the Baer DSM kit. Just like the Willwood DSM kit. Just like the Cobra rotor / VR4 caliper DSM kit.
The point here is that you now have 2 piston calipers. Also you have the larger rotor now, giving you a longer lever, helping apply more braking torque. That is the advantage this provides over the stock setup. Sure there is unused rotor space, but that's how these were engineered from Ford, and is how many other DSM kits out there are designed as well. Does this kit work, Yes. Is there possible room for improvement, sure, but that's the case with everything.
This is simply a budget kit, that damn near imitates the Baer kit.
Here's the same pics I had in the other thread showing the same situation on other kits:
1G Baer Kit.
Cobra brakes on an actual Mustang:
Willwood
VR4 caliper / cobra rotor kit