The problem of brake fluid boiling is simply traced to heat transferring from the rotors and pads directly into the fluid through the caliper pistons. When brake fluid boils, it releases air that is normally part of the molecular structure of the fluid. This air is compressible of course, and the brake pedal goes to the floor instead of moving the caliper pistons. Production cars tend to suffer from this on track days more as they don't usually have the extensive cooling duct and exotic brake materials as true racing cars.True racing only brake calipers generally come with titanium caliper pistons for one reason. Titanium as a material, has low thermal conductivity. This means that it is very bad at transferring heat. Which is good if you are trying to keep your fluid cool.
You can see that Titanium will transfer far less heat than steel and especially aluminum. For cars that are going to see hard or multiple track days, these shims are cheap protection from loosing the pedal due to boiling at the wrong time and making the day very expensive.