Thanks Jake. The old Hobie 18 (as opposed to the Tiger) had fairly short, wide boards to begin with and were pretty good heavy wind boats. Did you ever see the video they did of the three guys sailing it in Hawaii? That was wild!
Anyway, with racks and double traps, you should be able to sail that in pretty big wind, downwind, and it should be pretty stable too, especially when you pull the boards half way up. Not much chance of recovery once you put the bows under though, so try to avoid that! Keep your weight to the back if you are going to go deep downwind. But it is hard to learn spinnaker trim in shifty, gusty lake type wind. Try it on a day when the wind is more stable and the apparant wind won't move around on you so much.
Also, be sure to put some 12" casette tape streamers on your pole bridal wires and up on the shrouds so you can see what is going on with the aparant wind. I have found Brittany Spears or Clay Aiken tapes sound best when used for this purpose. If you were going 15-20 downwind, the apparant wind should have been ahead of beam reaching.