The air dams are there to help re-direct the bottom flow of air out and away from the underside of the car helping to reduce lift at high speeds, nothing to do with cooling and honestly not noticeable in a city driven car at reasonable speeds.
The air dams that help with brake cooling are actually attached to the lower control arms in the front and guide air directly to the brakes (performance pack). Also, if you ever install a spliter I am almost 100% certain you will have to remove the air dams but I am no expert in this as my car does not have an air dam on the underside at all other than the brake cooling.