They recently reviewed the standard C6 and the M5 on Top Gear - the M5 was 1:26.2 around the test track, the C6 1:26.8, I expect the Z06 would be quite a bit faster.

M5 review:
http://video.google.com/videoplay?docid=-5363968757069908445

C6 review:
http://video.google.com/videoplay?docid=-1218157444012227484

Gareth