Someone should just yarn their Z and take it on the highway. Along with a chase vehicle and a camera. I'd do it to my Z but it can't drive.
I think with a standard xenon airdam that is sealed to the radiator crossmember and the car low enough then stability can be dramatically raised.
Here are the plans for me aerodynamic work.
Standard xenon poly air dam sealed to radiator
belly pan to engine crossmember with splitter
bottom of wheelwells completely sealed
serious venting of engine bay through fenders
transmission tunnel sealed around transmission
rear diffuser (W/ fuel cell)
front opening closed by half
Nascar style adjustable rear spoiler
rubber side skirts
I think the most important thing to focus on (IMO) is what is causing lift at speed. It's not easy to add downforce to the front end of a Z by adding things onto the body. An airdam is about all you can do other than lowering the snot out of the car. Sealing the underside and redirecting the flow of engine bay air from going underneath the car to the side or top would have more of an effect at reducing lift than adding an airdam alone.
Just my .02