I still wouldn't expect any rolling road to be better than +/-5% really. So that's +/-5BHP on a 100BHP car and +/-10BHP on a 200BHP car.
The trouble is that power at the wheels depends on tyre pressure etc. But coast down losses are a very approximate thing. There is no rule that says the losses when the wheels are being slowed by the box are the same as when the engine is driving the wheels through the box. So take all RR results with a pinch of salt.
The most reliable method of measuring performance is timed speed increments or the 1/4 mile strip.