Depends on what stipulations you place on a car for it to be classified as a Z. If there was a drag car that was fully stripped out, tubbed with a huge diff and massive drag slicks, running a small block with NOS would that be included in your definition of a Z.
470rwkw is a 600rwhp car but there is a list (even if it is a bit old not sure if they are still updating it) of some of the cars. dyno list > http://www.pexcom.com.au/images/dynolist.htm drag times > http://www.aus300zx.com/forum/zxtras/qmile/index.html
Believe it or not 11.3 seconds is f'n quick and you wouldn't be saying that if you went for a ride in something like that PS: Anyone in Sydney with a high powered zed willing to take me for a spin, happy to give you petrol $$$. Fastest car i've been in was a skyline so i'm hoping to replace that
10.7 for the 1/4 by the UAS car (with 456kw) 517 rwkw from a S13 with a VG30 but thats not a Z 503 is next on the list but I can not see any details on that then the UAS car but as I said below I do not know how often the list is updated.