This is an interesting discussion.
I personally don't see Stage 3 aero to be similar to that of a high wing. The stage 2 and 3 aero on a factory C7 is a spoiler which will produce more drag per added lb of downforce than a wing, as it's running directly off the body of the vehicle. A high wing produces significantly more downforce (what was it, 300-400% over the Stage 3 Aero for the ZR-1 wing?) while producing less drag per lb of downforce. It's a lot more efficient way of producing downforce.
I have a friend who has a V8-swapped time-attack Miata. At Mosport (now known as Canadian Tire Motorsport Park), a former F1 world circuit, is an incredibly high speed circuit where there are only two low speed corners, the rest are balls-to-the-wall crazy high-speed corners. When he went from no aero to a big splitter on the front and wing on the back, he could easily maintain a good 20-30 mph faster through the corners and lost maybe 5 mph down the back straight (hitting speeds of 140 mph with an LS1).
That being said, 95% of people who are buying a Corvette with a high wing are never, ever bringing the potential out of the car. Very few will ever see a track, drag strip, or ran flat-out on a dried up lake bed. This was my biggest point. Have you ever had your Z06 up to 200 mph?