No, the Z06 actually has a higher drag coefficient (the higher the drag coefficient, the less aerodynamic) than the base model.
This is intentional. The Z06 requires more downforce to keep keep it stable at high speeds and assist cornerning. The only way to get downforce is to increase the drag coefficient of a vehicle, specifically in such a way as the forces push a targeted area of the car harder onto the track (so it grips better).
Rear spoilers provide added downforce to the rear of the car (the faster you go against the air, the harder the area is pushed downwards - this is downforce), which in turn puts more weight on the rear wheels, which is most handy while accellerating and cornering to keep them from slipping.
The front spoiler's purpose is keep the front end of the car stuck to the ground, otherwise it can raise at high speed and the steering becomes light and very dangerous. This was very common in late 70's supercars, when the technology to go very fast was around, but not a lot was known about downforce.
In some cases, the entire front end of cars have been lifted clear off the ground. The DeTomaso Pantera was one of the black widows known for this.
So, back to the aerodynamics of Corvettes: As demonstrated, it doesn't pay for a high speed sports car or supercar (hypercar in the case of the new ZR1) to be as aerodynamic as possible. That said, as far as cars go, there's nothing to worry about in the aerodynamics of any Corvette. The 2008 Z06 has a Cd (drag coefficient) of .34, and the base model has one of .31 - both are pretty aerodynamic no matter which model you're talking about. To put that into context, look at this list (long one when you scroll down):
http://en.wikipedia.org/wiki/Automobile_drag_coefficient
You might see that some of the 'vettes from a couple years ago are more aerodynamic. This is because they're not as fast as the new 'vettes and don't have as much downforce (because the newer ones need it). Also note the very high drag coefficient of the Formula 1 car - right at the top. This is due to the downforce created by all those fins and wings.
-Cowboy
