I also agree with blanks. More surface area = more braking power. Start to take away material and you have less surface, which equals less friction, which equals more heat put into the rotor.
Friction is independant of surface area.
Drilling and slotting takes away mass in favor of more surface area. (not swept area, which you are describing. Which is the area that the pad touches the rotor). Mass is what stores heat, more mass equals more thermal storage. So you are sacrificing thermal storage for slightly better cooling. (among other things like duribility, and imo, safety) . It is a balance. If you are looking for better cooling, getting a larger rotor would increase mass and surface area, and is a better choice in my book.