The way I interpret the function of the dual-pitch prop is that they *should* provide more thrust when the plane is flying slowly (such as on take-off or when climbing steeply). They do this by ensuring that the root section of the blades isn't stalled like on a normal prop.

When the root of the blades is stalled they contribute very little to the thrust but absorb power from the engine.

A dual pitch prop *should* allow the engine to spool up more quickly than a constant-pitch one and that should mean that your engine produces more static thrust so that the plane will accelerate more quickly and climb more steeply.

The downside will be a performance hit at high-speed.

Or I could be completely wrong :-)