I think that was looked into in one of the many Titanic documentaries that have been made over the years. From memory I think it was stated that a head-on collision would have caused massive damage to the bow of the ship, but at worst only 2-3 water-tight compartments would have flooded. As the ship could stay afloat with any 4 compartments flooded, the ship most likely would have limped for the rest of the journey, but would probably have survived.