the change from GT brakes to cobra brakes was an amazing one.
with GT brakes get up to about 120 then try and slow down ot 60, now try and slow down again... youll notice that they hve already faded.... now do the same with cobra brakes... ok now peel your face off the windshield now....
Those were my observations as well.
I agree however with the whole "New parts = better braking" statement. I hang out on the Fox boards since i have a 5.0 and Fox Mustangs have terrible brakes. It never surprises me when someone puts on some cheap drilled rotors on the front and claims it made a huge difference. Any new part will outperform factory worn stuff.
However, the different between the cobra stuff and Gt stuff is not the same. I took my GT brakes off with 20K miles, so they were not very worn. I still have them and the pads are still full and rotors are well within spec. In normal street driving they are the same. A stop from 30MPH is going to feel the same if you have 9" or 17" brakes.
However, i noticed the difference from high speed stopping, especially coming off the highway. I would be doing 65MPH on the highway and get off the ramp and lay into the brakes hard. With the GT brakes, they would bite good at first, but when i get down to around 45MPH if i need to increase pedal pressure to slow more (fur a turn) i would notice they would be starting to fade slightly.
With the cobra brakes, i would never get this sense of fade. I would add more pressure late into the hard braking and the brakes would respond just as well as they did on the initial bite. I even notive with the stock PBR pads they repeated stops actually makes the pads bite slightly harder.
That's the main difference between the GT and cobra brakes....MASS. Forget diamters and just figure out the mass of a 10.8" x 1.0" rotor vs a 13" x 1.1" rotor. The larger rotor can absorb more heat and resist fade better.
More massive rotors also slow you down a tad bit as well do to rotational mass just like larger wheels
