The 18m proa would not benefit from a vertical component from the board below 20kts. 

The HP with kite loses one of the features of a HP with a tall rig because the kite offers little healing moment.  That means the windward hull is not being unloaded and remains high drag as wind strengthens.  In that case a board offering a vertical lift component at the ww hull can offer a benefit at lower speed.   

Bucket list top speed benefits from a canting foil near the beams with only the forward one immersed.  I have attached polars that indicate it could get above 30kts on a broad reach and offer a speed advantage in wind above 10kts.  The downside is that they reduce windward performance because the vertical lift generated adds to the heeling moment; it helps windward performance in light wind because it assists the rig in reducing the displacment of the ww hull.   For windward performance in higher wind, they would be better being mounted to the lee of the lee hull but then it is getting more like a trimaran without the lee ama.

In this analysis I got the best result with only the forward board immersed.  From memory the best location was 3m forward of centre.  In that position, almost the entire displacement is carried by the single board when the boat is on the limit (so big loads).  The hulls play almost no role in the hydrodynamics - ww hull flying and trailing end of the lw hull bouncing along on the plane providing flight stability.  It could be quite twitchy though with propensity to porpoise.  There is weather helm so the trailing rudder is needed to counter that.  This arrangement requires 4 appendages; two large canting boards with the leading one immersed and two small vertical rudders in the ends that could both stay immersed.  The front one would have reduced immersion when the lw hull was flying.  It may require T-rudders to provide flight control but I did not get that far into the dynamics.  

Rick