We've been playing with this card on a 780i board and the Q6600 and also ran into a few performance issues in both single and SLi.
Running quad SLI saw terrible performance really and although it may be bottlenecked somewhat, the drivers and scalability still need optimising we feel.
We're doing further benchmarking on it tomorrow so hope to bring you some charts and actual figures for Quad SLI, hopefully on a 790/Extreme setup.