If the Z/28 is beating the ZL1 by over 3 seconds a lap at the proving grounds in its current trim and testing stages, it will beat the ZL1's ring time by over 18 seconds.
ZL1 was 7:42.XX
Z/28 will be 7:23.50 or better.
I'm going out on a limb and gonna say 7:22.74
UNDER CONSTRUCTION. ETA 2017 ish...