You might be interested in our trim-tab design guides. We've tried all different kinds of designs and tested them at sea.
http://www.sailsarana.com/selfsteering.htm
We've used our trim-tab for about 7,000 miles and it's worked great with the autopilot. Hardly any power use, there's no load on the tiller pilot and it has built in feedback which keeps the boat on course without the autopilot having to fight the motion.
Eric