A GPS unit will be pretty accurate as long as it can see the satelites and you don't jerk it around too much. Unfortunately, when you are on a catamaran going upwards of 20 knots, you will be jerking it around and it will not have a good look at the satelites all the time.

I once threw my handheld from one side of my cat to the other (on the beach while rigging, to my crew) and it registerd 53 mph. I just tossed it easy, underhand, so I know it wasn't going over maybe 10, but because it tumbled I think it decided it must have gone 53? The point is, in your car going down a smooth road in a straight line, sure it works great. But on a picthing cat on a rolling sea, not so accurate.

Here is what you can do: Have your crew hold the GPS unit somehow that the antenae will have a good look at the sky for a few minutes while you sail upwind, bear off and set the kite, now have him get out on the wire with the kite sheet in one hand and the GPS in the other (better tie it to him) and now you heat it up and get it going as fast as you can for as long as you can, and have your crew read off the speeds as you go. You might see 20 knots on an F18, just before you swim. (insert picture of the Hobie Tiger pitchpole during the speed run in the French Ditch). I have found a handheld GPS will only give you accurate numbers in a steady state. Once you start tacking and gybing if the boat is bouncing all over the place in waves, the "max speed" number is no good. At the end of the day you can use the distance and time it records and do your own math but that also includes all the slow sailing while tacking.