You also might want to consider how much hp the Vette was actually making. Does anyone here think that GM in 1999 wouldn't pump up the numbers to increase sales. GM doesn't exactly have a track record of honesty when it comes to..... well.......anything
In other words......... maybe the Vette wasn't making that much hp and todays Z actually makes as much or more than that Vette really did. Which would explain how in this case, it is possible for the car with the 'lower numbers' to be faster.
Quarter mile and rated hp numbers have always been more important to GM when it comes to selling a American hotrod like the Vette or Camaro. I would question GMs numbers before I would question what came up on the timers when a independent tester ran it at the track.