I always take published
I always take published performance figures with a grain of salt. When exploring the ultimate capabilities of a car, track condition, air temp. and humidity, even wind conditions, which can vary minute to minute, almost certainly day to day and most certainly year to year, can make a pretty significant difference, easily a few 10th's if not more. Then there is the consistency of the driver(s).At the track I can't ever remember seeing a driver, professional or otherwise, run the 1/4 or a Road course more than once and have identical times, close, but not the same.
I have run several cars at our local track, I consider myself to be an above average driver, and for example, running my Z on a 70 degree, low humidity, calm day, results in quite a different time than running it on a 90 degree, high humidity, windy day. At best published numbers put you in the Ballpark and is only an indication of what she will do.