Did one with the WR1OC at WRC @ Silverstone recently, was very interesting to see how the cars varied. I would say though that the RR dyno's produce differing results depending where you go. Some of the guys had been to Power Station and got good results a few weeks earlier but on the WRC rollers were 20 - 30 BHP down on the Power Station readings, so a 'True' reading is unlikely I think. But its a good guide