Bit out the blue but Top gear did something like this ages ago. They all bought supercars on a budget and before they thrashed them they put them on a mobile rolling road one of the cars loast about 100BHP!
 
like someone has said, it really depends on how the car has been looked after, as i know, some clio valvers are booked at 137bhp and some are putting out over 140 with no modds, and its plainly down to for you drive and how you look after the motor.