It would depend upon where the hole was, of course. Near the bottom a 300 foot wide hole would certainly affect the dam's integrity but might not compromise it to any disastrous extent because of the thickness of the base (remember, it's rather wedge-shaped). It would also make a difference on which side the hole first appeared -- it would be far more of a problem on the upstream side, as water, being incompressible, would hammer against the weakened structure after being "blown back" away from it and thereby cause greater damage than the initial hole would have been. For more information you might check out Operations Chastise and Catechism, carried out by the RAF in WW2, and the operation of the limpet mine.