Somehow i do not think that the Vikings used sunstones to discover anything except to maintain a rough heading. Any direction as long as it is toward the setting sun lands you in America. Even then you are merely sailing in a primary current which also gives you orientation, perhaps not safely the first time around but certainly thereafter.
Testing here shows us that in the worst conditions, a sunstone becomes useless and at the level it becomes useful, alternatives likely work as well or better. Thus in the end it is a promising curiosity among others that were used for navigation. I suspect that the lodestone was just as handy as well and its antiquity has never been defined well. Certainly knowledge of the lodestone appears to have come from the North.
The key to discovery was always to find a way to sail into the current. That way escape was easy. If that was impossible then you really rolled the dice and hoped you had sailing power sufficient to retreat. Recall that the viking ship was a wonderful sailor and could likely out sail anyone else. This made it ideal for exploration. It is also a reminder just how brave Columbis was to sail as he did with what he had.
An example of a Viking sundial. Credit: Soren Thirslund