Front-only upgrades may feel like they stop faster on the street. In fact, brake upgrade suppliers always tout shorter stopping distances. If brakes don't stop the car, how can this be? Until you reach limit braking (where the tire is 100% in control of stopping distance), the stopping force generated by the torque created at the rotor dominates the available stopping forces. With bigger rotors, you can generate maximum braking force with less pedal pressure, less pedal travel, and therefore less time. It's this reaction time and the faster ramp up of the torque applied at the tire that creates the shorter stopping distances. When you're traveling at 60 mph, you travel 88 feet per second. If your brakes reached peak torque .1 seconds faster, then you just reached limit braking 8.8 feet sooner. So in that respect, bigger brakes can shorten braking distances. Another way brake upgrades feel better on the street is that the bigger rotor will generate greater torque for a given brake pedal pressure. Therefore, the driver can press lighter on the pedal to get equivalent stopping power. It feels like the brakes make a big improvement in stopping power -- but they really don't. You just don't have to push as hard to get the same power.
For the weekend road racer with a street car -- in a word, heat!
Stock style brake pads are not designed for the sustained high temperatures created by repeated high force braking. Their coefficient of friction drops significantly, and the material begins to fall apart with the high temperatures involved. On a track with several hard braking zones, brand new stock pads can wear down to the metal backplate in two hours of track time. Racing pads will last 4-to 6 hours of racing time, and their co-efficient of friction will remain consistent at the high temperatures.
Stock rotor sizes, likewise, may not have the ability to dissipate the heat generated by race-duty braking. The repeated, high-temperature heat cycles may destabilize the mechanical integrity of the rotor. It could warp, or generate numerous small surface cracks. Larger rotors act as larger heat sinks. Their larger mass reduces the peak temperature they reach, and they stay mechanically stable. In a nutshell, they last longer.
For the part time racer, racing brake pads are almost always needed for road racing. Rotor upgrades depend on the car, and it's original design purpose. A Vette's rotors will be fine. A Honda's rotors may not. Whether you need bigger rotors depends on the track you're racing at, and how hot the rotors get. If they warp after every couple race events, then larger rotors could save you money in the long run.
OK, so after reading this, the experienced racers among you are saying "yeah, but..."
There are conditions when this "keep the balance the same as stock" conclusion does not hold true. This is when the makeup of the car has been dramatically altered from it's stock form. Primarily this involves major changes in the car's CG position, it's total weight and / or weight distribution, or in its weight transfer characteristics under braking.
If you have gutted the car, outfit it with much wider than stock tires, or use very sticky race tires, the net result of the braking balance can be quite different than the stock ratios. The ratio of front and rear braking forces changes typically reducing the amount of rear brake force required.