The optimum ama shape for paddling and sailing is very different. The photo above shows a 45' Hawaiian racing canoe rigged for paddling only. I've drawn in a dotted line along the waterline to show how very little of the ama is actually touching the water when in use. (Click the photo to enlarge) The ama is also quite short in comparison to the length of the main hull.
An ama like this would not be a good performer under sail not only because of its lack of buoyancy but also how the entry is shaped when it is pushed down by a press of sail.
The above drawing shows the ama and main hull of the Va'a Motu. The ama is stretched right up to the main hull bow to give the needed buoyancy to prevent a diagonal capsize. The entry also has very little rocker to make it pierce waves rather than pitch over them. Constant changes in pitch or "hobby-horsing" plays havoc with the flow over the sail and makes you go slower.