Hey Gary, that is a cool pic, looks like the sensible one was out of the sun, under the tramp !
In answer to the question "How fast are they " ? Well, over 40knots is what the guys are doing at the speed trials, although I wouldn`t try that in my living room.
On the subject of windstrength your average kiter goes out in, 18-20knots is max, when it gets over 20knots only a few experienced guys keep sailing kites, over 25knots and they are all packed away, here anyway. Then it`s the windsurfers who come out to play. From what I`ve seen the windrange for kiting is around 14-25knots.