Bit of a trek but if you're able to be anywhere near Castle Combe you could give Circuit Motors a call. Haven't popped by in a quite a few years but Paul used to operate it who's a real enthusiast and will not fiddle with the figures.
As others have said it's best as a bit of fun and preferably with other Z4's on the same day to compare with. In the past Paul has organised a few events the same day Castle Combe have run their turn up and drive trackdays so you can watch or join in with the track action after a run on the dyno.