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.
Is more approachable than one might think, as you can currently find two of these for less than 1,000 USD.