This is rather old topic, but I think this link will provide some answers, however not all of them:
Evo 8 (very little aero, mostly marketing, but shows some pics from underside):
http://www.myevo.com/ecms/Lancer%20evo%20VIII.pdf
Current model:
http://www.mitsubishi-motors.com/cor...008/20e_08.pdf
Of course one must remember that this vehicle is not designed for low cd, but for downforce and good cooling when driven hard, so while some elements are designed reduce cd those are mostly meant to reduce increase that comes when making downforce and as such may not apply to lowering cd purpose as they are.
I would think that with hood vent one could get away with smaller front opening for radiator and it might make possible to cover underside of engine bay better.
But what it do for air that goes to wheel wells, will it cause some issues there?
Also I have no idea how vent would affect topside aero, I would imagine that it might increase pressure at windshield and perhaps increase air that goes to sides and the top, where it might cause effect of mirrors and rear top and sides being larger than without vent, but it can be whole lot different from what I think as aero sometimes is not quite so what you think it would be.