Yeah, that's strange how an auto Z is faster than a manual. When i bought mine i thought it was exactly the opposite! Stock, the twin turbo is really not all that fast. Not until you get to stage 3 in them do the really wake up.
I drove a z before i bought mine and it was stage 3 and i didn't know it. I was like "Wow" think what this will be like when it's modded. Then i bought mine. On the way home i had to get out and check to see if it was really a twin turbo as it didn't feel ANYTHING like the one i drove before. Of course mine was stock.
I'm somewhere around stage 4-5 and the car is just unreal. There's no comparison to how it drove before and now. I can't imagine what a 600+ hp Z would be like.