Color me surprised.
I drove the 9-5 for a total of 40 minutes at 70-80mph today and the HD lip held strong. There weren't even any ripples in the lip when I finally parked the car. I also noticed that my new lip was very similar to a co-worker's stock lip on his new Mustang.
The lip sits about a thumb's width from a standard concrete parking block.
I was planning on keeping the lip for a month and then removing it. After today I think I'll keep the lip installed until it wears out. We'll see how it holds up to rain, snow and ice.