It is dangerous going so fast without airdam and rear spoiler (lip spoiler) inspired in NAscar.
Almost try to get lift 0 or below 0.
Your car will be unstabble at high speed if the wheight distribution with the Gravity centre in front and the pressure center in the rear. some lateral wind and the car will crahs.
maybe page from 50 to 55 are useful to tune the wings with a compromise between drag and lift:
http://www.seat-sport.com/2013/cars/UMLeonMK2.pdf
Audi TT without rear spoiler had stability problems.
We Take Audi's Tamed TT for a Spin - First Drive Review - Car Reviews - Car and Driver