Dont forget that you actually still want the air to be able to leave the engine bay from the bottom of the car, if you seal it to well it will actually create lift on teh front end and hamper your handling not help it.
that said, if done correctly it will help remove air from the engine bay causing low pressure in the bay and helping downforce (or atleast decrease lift)
use this time to seal from the front bar to your cooler, then from cooler to radiator to increase efficiency of both. also forces air the go elsewhere Ie to the sides, which will also increase the efficiency of the front end and reduce drag.