I think when you are talking Cobra and Z06 you are talking about two seperate animals. As a straight line car, a true 427 Cobra will beat the STOCK Z06 everytime. We're talking about a high HP engine in a 2400 lb car for the Cobra. 385-405 HP for a Z06 at 3100 lbs.
This interview may give you some insite on the direction which Corvette/Z06 is going.
http://crossedflags.com/media/Dave_Hill-20030830.wmv