They MAY have a slight effect towards keeping the front end down. Alot depends on whether or not a high pressure 'bubble' builds up behind the airdam and in the engine compartment at those speeds. If it does, the vents can help relieve it.
The primary intent is better cooling. Not just for the engine, but the alternator runs pretty hot, too.
Keep in mind:
Half my front end lift was tire related at 125-130. I fixed that with new tires, and have an even newer set now.
I also have a new feature on the car since my last 140+ test run. My lower scoops for SC cooling and CAI. They should provide a little front downforce. Yup. It'* been that long since I went that fast.
In addition, when I did my new rear shocks, I adjusted the ELC for slightly higher rear stance while the alignment was being done. This forces my front end down slightly. It also cuts my front ground clearance pretty close with the scoops down there. I may have to move the scoops up into the front airdam in some manner.