The speed advantage is easy to explain (as I did) and may be mathematically proved (by someone else <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />), but it is a small advantage in theory and even smaller in practice.

Drift angles are in the range of 2 to 3 degrees and the asymetrical or angled foil can not eliminate all the hull's contribution to lift. Drag INCREASES if the foil's angle is too big (or the section too cambered) for a given speed, so it is safer to use a small angle of incidence.

Ideally, the angle of incidence should vary continuously from close to 0 to more than 2 degrees as speed increases (also depending on the section's camber).

In practice this means that a compromise is necessary and that "ideal lateral lift" - obtained exclusively from the foils and not from the hull - is achieved only at one definite boat speed.

I'd expect a boat with angled foils to excell at that specific speed, have a marginal advantage at higher speeds and a disadvantage at lower speeds. This probably explains the situation you observed.