Looks like it's in the 2-4% range on average. Some oddballs prefer PCIe 2.0 on the Nvidia side, which is odd.
The higher the performance of the GPU the more data it needs and the more the bus can bottleneck it, hence my expecting these cards to fare a bit worse than the 6xx series.
EDIT:
Bitcoin is SHA encryption (integer stuff), Nvidia doesn't have anything that I'm aware of that can come close to matching AMD's cards.