Well I've already got an Etrex Legend, looks just like that one you have, I wonder if it will work with the Kattack. I usually tape it to the boom or tie it to one of the hiking straps at the back of the tramp, out of harms way. I've seen as fast at 17.6 knts. going downwind solo, with the kite.

I know I've gone faster on a blast reach with my (195 lb.) son on the wire, we didn't have the GPS on board that day but it was scary fast, it felt like a windsurfer skipping over the waves.