If you go to
http://www.mulsannescorner.com/ there is a list of trap speeds from past Le Mans races (scroll down left side of the page). As to why it's slower, I'm not sure of the technical explanation. But in my opinion the downforce that Audi designed into the car is purposeful and not an accident. I don't see it as a flaw, as you seem to suggest. They know the fast way around Le Mans and it doesn't have anything to do with having the best trap speed. They want the lowest lap time and that's what they are getting.