Heat sinks only work when in direct contact with the heat source they're reducing. The idea is to keep the fluid temperature level to a minimum to prevent boiling, and to keep pad/rotor temperatures low enough that your friction coefficients don't drop below the optimum threshold.

Now, if you have good track pads, you're more than likely going to start boiling the fluid before you exceed the temperature threshold. You could even be solidly in the middle of the "comfort range" for your pads, and still start to boil the fluid.

It's all relative to your system of course, you have to decide where the air will do the most good, because like it or not, heat is dissipated from the caliper.

On various road race cars I've dealt with, we seem to get the greatest benefit by giving the brake assembly as much "head-on" airflow as possible. Vented rotors are going to draw air through them naturally, so getting rid of the dust sheild helps that a lot, but the airflow across everything and hitting the caliper does make a difference. Of course, the better the fluid you use, the less of an issue caliper temperature is as well.