I maxed cunning over endurance on my bird and it was the best choice I ever made, however, I do not use super sprint. I'm curious as to which SS build you are talking about- one that simply incorporates it as one of the two abilities the chocobos gets, or that fringe build that was designed to let you super sprint for the entire race? I'm pretty sure they have different stat priorities and the importance of endurance will probably vary depending on which it is.