From an automotive background, I would consider a 20% loss in horsepower to the wheels vs. rated to be a good guess...
That said, I'd happily pay the $50 to run my cub on a chassis dyno, but the margin of error of an automotive dyno wouldn't even touch the rated power of our cubs.
On the worst day ever, I'd say 10hp at the wheels... Probably closer to 12hp.