In the absence of a rolling road it is nice to see if the performance of the car is dwindling in horses in any way.
There is an excellent on line 0-60 mph calculator http://www.060calculator.com/
For the J15 this gave 4.31s for 627kg and 155bhp. The weight was taken from the recent corner weight measurement and the bhp was taken at Sitech racing last year http://sylvabuild.blogspot.co.uk/2013/08/ecu-re-mapping.html
I set up a track on Race Chrono, using a safe bit of road where the terminal speed was less important and 60mph was obtainable in relative safety. You can see that the example gave a result of 4.8s which is about 0.5s pesimistic. However if you take into account that the test track was up a gradient then an error of 0.5s is not bad correlation. Also 0.3s is being lost in gear change time, something I may or may not be able to do something about, it is always going to be slower than a paddle shift in the Megabusa.
I have measured my wife's Mondaeo, Westfield Megabusa and a Westfield Zetec (*) and all gave results that were within 0.5s of prediction (* butterfly fault).