I have no dog in this one either like Zip says. Maybe my thinking may be oversimplified due to my lack-of-knowledge about dynos, pricing & building race motors. But......
I do believe that a man should do what he says he is going to do however. If the motor cannot make reasonably close HP as agreed (including +/- dyno variance) why should buyer be satisfied @ 947HP even if the motor cost $1.00??
That said.....
It is really no different than how my company builds bows. Our dyno is a "chronograph" instead. We advertise a speed on each model based on an industry standard ("IBO" 30"/70lbs @ 5grns per lb & +/- 8 FPS). If a bow does not make speed within 8FPS, it is taken apart, tuned, rebuilt etc.... until it does... period. The motor may be a good deal @ the price; but it is not the deal buyer signed up for.
In spite of that......
Disputes arise in everyone's equipment/measuring with chronographs, scales, dynos, etc... but 143HP variance is BIG ... that is 13% less HP than advertised.
My Opinion: If an acceptable dyno variance is say 4 to 5%, then seller should compensate for the lost 8 or 9% agreed HP to buyer for example.
Am I oversimplifying this situation?? Am I missing something?