My experience is that seastate is alot more important in deciding whether to fly the spi then windspeed. I sail solo and I make a point of pulling my spinnaker in any windspeed. My boat is more controllable downwind with a spi then without. Last August I finally encounter conditions were I couldn't fly it properly and I flipped it several times in row. The main problem were the very tall, steep and fast moving waves. That and a set of rudders that just stall too easily. Wind was a force 5 with gusts to 7 (20-28 knots). Other then that I have always been succesful with the hoisted spi in a blow.

Spinnakers don't add much stress to your boat. They most stress out your mast top if you don't know what you are doing with your mainsheet.

Hooter are a different matter. They can stress up your hulls significantly more.

Wouter