It's not hard to beat them, most people go there to hang out
I think the Cayman S and the Z are going to be relatively close (within 2 sec), but it's hard to compare without a professional doing it with equal equipments. I think track would make a difference too since the Z has HP advantage while the Cayman is lighter and smaller. I drilled a video from youtube, I found my times to be very similar with his. Of course I don't know exactly what it has, but I do know my car was quicker down the straights.
http://www.youtube.com/user/CaymanClub