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All those nasty scoops and vents that are festooned (good word, eh? Look it up if you don't know it already) all over the car make for most of the increased drag, I bet. Generally, the "cleaner" it looks the lower it's drag, and the Z is definitely less "clean" than the coupe. But "clean" generally isn't very exciting, design wise (think Prius). Given both the C5 and C6 Zs have similar numbers, I'd guess GM decided that about 0.34 was an acceptable "budget" for the CD and they used all of the allowed budget hanging gee-gaws off of the car to make it look better (and while I don't like the C6 design very much, I do have to say that the C6Z looks tons better than the plain-jane C6).
And yes, the reason for the C5 Z's lower top speed is the difference in CD that results from going from a hatch-back to a notch-back top. The extra HP isn't enough to make up for the higher CD. But (for the C5, at least) the notch-back design adds stiffeness to the chassis and so the car handles better. It was an intentional tradeoff.
It's interesting that GM decided to stick with the 3-door configuration for the Z this time around.
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