According to Mr. Wyndham's interview with John Fitzpatrick (
LINK) the mysterious course where the z/28 proved to be 3 seconds faster than the ZL1 is the MRC, or Milford Road Course.
Quote:
Originally Posted by Mr. Wyndham
The 1LE already proved faster than the Boss 302, which is no longer sold anyways. So were you competing with anybody with the Z/28, or did you just want to show off?
Let me put it this way. The Z/28, in our initial testing at the Milford Road Course (MRC) is three seconds faster than the ZL1. The ZL1 beat the GT500 there by three seconds. You can do the math from there…

Now someone correct me if I'm wrong, but I believe this course is owned by GM.
It is a 2.9 mile course with 18 turns..
I am having a hard time finding lap times for current production vehicles, but this gives us an idea of how much faster the Z/28 is than the ZL1.