I do believe the C6 shown is displaying more the results of the slight rear downforce (negative lift) the stock body has inherently designed in, combined with the almost neutral behaviour of the front vs actual lift generated.
From my recollction, the C6 has a close to 0 front lift number and a slightly negative lift (downforce) in the rear.
C5's and C6's have been doing 200+mph runs since 1997 and it was apparent from day 1 that the car is relatively neutral and responds really well to researched aero additions.
Remember even that F355 while appearing aerodynamicly unstable has a flat underbody that helps in preventing such disastrous aerodynamic events. And another thing, in order to flip the car over, the positive lift will need to overcome the weight of the car - so lift would need to be in the +1000lb range to yank the car over.
Of course these lift number will be dynamic of a panel begs to rip off or the car veers left or right and suddenly the body may take on a aerofoil profile as it rushes through the air resulting in really chaotic behaviour - but I dont think the standing mile even at 240mph is enough to induce the funky behaviour we saw in that video - or not
