Ideal is 12 to 15. It's still "pleasureable" for me up to about 18 knots but beyond that I'm starting to worry about sail wear and equipment survival. When it's getting really gusty (22+) and capsizing starts to become less optional, I'm not enjoying racing anymore...but then again, we have a pretty good track record staying upright in the crazy stuff and I'm not often sailing in those conditions so they're pretty foreign to me. Perhaps some more time on the water in the heavy stuff and I'll gain more confidence in the equipment and perhaps rich enough to not worry about the sails!

Even sailing in 18-20 I think the kite helps to settle the boat down and gets you moving faster relative to the oncoming gusts (so they hit you a little softer)...of course, it does mean that you're going that much faster if you crash and burn.