The vents definitely help to cool things off, especially since I've removed all of the heat shielding. When the car is sitting still you can feel the heat coming out of them. There's a low pressure area right there created by the spoiler, so I'm assuming that at speed heat continues to move out.
For fiberglass parts, try Getty Design or MA Shaw. One of the two of them, I don't remember which, made my front bumper cover, trunk lid, and wing.
I'd love to know more about your door mods...