The yan chi dao is a variant of the willow leaf dao with a clipped tip. The better examples look like the ones you have posted, but a simple clipped tip is more common. According to Scott Rodell, the "goose wing" name is a Chinese collectors term, and he seemed to think that the name "phoenix wing" appears more often in the regulations.
The clipped tip brings the point in closer line with the hand making for a slightly improved thrusting ability. You have noted that the curvature is more subtle than that of most willow leaf dao, and a slightly reduced curve would also improve the thrusting ability. I do not see them as similar to yan mao dao in design because the reduced curve of the yan chi still makes them different form the yan mao which really only curves at the tip. Overall, I see them as a way to combine thrusting ability with a curve, so in that way they are similar to a yan mao, but still they are in the willow leaf camp.
Josh
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