I really do attribute much of my usual "better than average" mileage to driving mostly around 70 mph, having a 6 spd. and keeping my wing in the down position. I more often than not have my drivers side window down, and the AC off as well, and hardly ever drive in the lower gears unless I'm running below 35 mph.
Since I seldom drive much over 70 or 75 mph. I really don't think the lack of down force (or spoiling effect) caused by a non-deloyed rear wing puts me into any negligible danger as long as I'm on a smooth dry highway. If I'm ever caught in a rain storm, I always put it up while travelling at highway speeds.
I'm probably one of only handful of Crossfire drivers who have actually driven their cars over 100 mph with the wing down (just last Friday on the PA Turn Pike as a matter of fact) and I honestly didn't feel any difference in stability when I raised the wing (to silence the warning chimes).
I don't recommend driving like that, but sometimes it becomes necessary just to not be left behind while driving in formation.
I can promis you though, if you ever get into any sort of accident with your wing down when it should have been deployed "by-the-book", YOU will be held accountable whether it had anything to do with causing the accident or not.