I did the plastic headlight with turn signal swap so I could use the turn signal openings for the brake ducts about 10 years ago. Currently setting the ducts up on each side of the radiator as shown in the pics below. The turn signal openings are way out at the side of the car and air is moving fast horizontally due to bow wave with a splitter. Also using air from right above the splitter may reduce air pressure on the splitter a bit (less downforce). With the duct openings in the core support the tubing is shorter (less weight) and less convoluted also so flow should be smoother. Ideally the openings in the core support would have a more rounded curved entrance but I was using what I already had.
In the lower pic you'll also see a hood to core support seal, bumper bracket seal, and core support to fender seals. So the only air that gets past the core support is through the radiator or the brake ducts.