I've been doing some 3DMark benching and I was able to overclock pretty cleanly to 785/1100 and I got a score of ~16.3K
ATITOOL's been working like a charm for me. I have the XFX Alpha Dog Edition.
As for getting to 850, it seems possible to me, as I've gone over 800, but then the artifacts seem to become an issue real quick. It's different from card to card.
The people at Hardware Canucks (Canadian site) were able to overclock to 832.
I was a bit doubtful of this card, but I am extremely satisfied at this point. No way you'd see a GT even come close when overclocking. The price difference is worth it IMO.