Hi Eric,
I have spent the past twenty summers on Maui teaching high level windsurfing, and though I have only kited a few times (not enough time or money to do both), I will pass on my non-medical thoughts. First of all, I begged, pleaded, and whined for Dr. Gross to give me permission to windsurf at 3, 4, or 5 months. Lots of excuses like, " I am really experienced, and I promise to take it easy". He just kept looking me in the eye and saying SIX MONTHS. He insisted that a couple of months off the water was a small price to pay for the lifetime new hip he had put in. The alternative if I blew it was to come back to him for a revision.
He made two points that really hit home with me. In all action sports, s**t happens (my words, not his). A fluke gust of wind, a failed release mechanism, or somebody else doing something stupid; the things that you think will never happen, can happen fast (did you ever see the video on You-Tube of the guy whose kite was caught on the wing of a jet coming in for a landing; 20 to 180 in under a second!) Anyway, the Doctors point was, that at under six months, our hips are not ready for the sudden and unexpected.
The other point had to do with what he called "micro-motion". It does not require a big "oops". A series of small impacts can cause micro-movement at the bone/ prosthesis interface, that can inhibit bone ingrowth, and if this happens enough, the bone may heal without solid attachment to the prosthesis. I know that in windsurfing across choppy water, there are constant micro impacts that would fit this description. I hear your argument that kiting at under 20 knots can be fairly smooth, but I can just see Gross looking right through another one of my rationalizations, and repeating SIX MONTHS. I am sure that your bone growth is mostly complete, but that is not the same as totally complete.
Anyway, for my part, I spent all of last summer living on one of the best windsurfing beaches in the world, and I never touched a sail. I might have gotten away with it, but I am glad that my Doctor was willing to confront my stubbornness head on. Now I know that I can hit the water full throttle next summer, and for many more.
(P.S. Before I moved to Maui, I spent a few years on your island of Kauai, helping to run one of the first windsurfing schools down at Anini Beach. I have many happy memories of wave-sailing at Tunnels, Mahalopu, and Ship-wrecks.)
John