As Choke says, 8 knots is about the maximum for the Hobie sail. Which makes me wonder why they made it like that.
I have had the Hobie out in a good 20 knots with the Spirit sail and the excess wind just spills over the top of the sail. There is no danger of being flipped by strong gusts of wind. If you want to you can harness that extra wind and hang on for a good ride but you can also release it by allowing the flexible mast stays to bend like a fishing rod.
I'll get some pics next time we go out sailing. Just a bit busy building a new deck at the moment!
Cheers,
Jake