I'm having the same
problem... On my RR, I got 169hp on the flywheel, and 121hp on the wheels (loss of 58hp!)!
It was done in 5th gear... Could that be the
problem maybe? Is it supposed to be done in 4th gear? (the guy that RR'd it said that if he does it in 4th, the session doesn't last as long as it should!)
Been 2 times to that RR, and the figures were the same. A friend went there with his Fabia vRS and got 129hp on the wheels...
But the thing is, that my car goes the same or maybe a touch better than his!