I can't speak to the suitability of the largest electric unit (16,000 displ) on your boat. I did use this size unit on my Channel Cutter which had a 14,000 displ. and full keel configuration. I believe the unit was a Simrad..not sure if they are still around. Under the conditions you describe I would not think that size unit (electric) would last that long.

I am thinking that at some point the sea state becomes as much as issue as the displ. I would suggest that you check the manufacture with you application and see what they suggest. If I had to decide I would think a hydraulic unit would take the abuse much better. The motors in the Tiller type autopilots are rather small and geared quite low so getting a quick action on your tiller in such a sea state may not be possible. My 16,000 pouind rated unit lasted about two years under much less strenuous conditions of motoring. I would be curious to find out what you decide and what the manufacture has to say as well. Best to you

Mark SV "Grey Seal"