Lots of misconceptions here as to what a "warped" rotor actually is.
On a normal street car that has factory brakes and it's not being used strictly as a track vehicle, it's not possible to heat the rotors to a point that they physically warp. What causes that pulsation in the brake pedal is microscopically uneven brake pad distribution over the rotor surface. As the brakes wear, some of the pad material will deposit itself over the rotor surface - again - on a microscopic level. When the deposits are made unevenly across the surface, you get that vibrating brake pedal.
In my personal experience, the best way to avoid this - in most cases - is to use a quality brake pad. Some pads are plain crap while others may not be designed for the type of spirited driving you are doing, even if it's just a few twisties here and there.
The brakes on the Sonic feel great 95% of the time, however there were a few instances where I was braking from about 90 for an upcoming hard right and the brakes have lost their bite until I was slower and they were cooler.
Solution: once it's time to do the brake, I'm going with Hawk pads and expect the issue to disappear.