I just found out about these last week and haven't received the hardware yet, so I can't give you real numbers. That said, one can probably expect at least a 10-30% penalty when the cards need to communicate with one another. Other workloads that don't require constant communication between cards can actually expect a performance boost. Your mileage will vary.
Cool, I guess… If you have tens of thousands of $ to drop on a GPU for output that’s definitely not usable in any 3D project out-of-the-box.