I've fitted Doelfins to three different RIB/engine combinations. In each case the boat got on the plane at a much lower speed - particularly useful in Portsmouth harbour with a 10 knot speed limit - and didn't seem to have to work so hard to 'get over the hump'. Without the foil, the combinations felt very back heavy in comparison. I also have the impression it helps the prop grip in tight turns.
You've got to be brave to drill those holes in a new engine though!
Nigel
"There is nothing - absolutely nothing - half so much worth doing as simply messing about in boats."