I am not an engineer. As far as I understand they found a way to do some over-clocking combined with the bigger fan.
As soon as I test them I will provide screens. They push it to 420 - 430mh/s per board, so with 4 boards 1.7G is possible.
PS: Yeah, the ROI is always a problem since the beginning of this year ; )
The custom mining thread is full of posts about Spartan 6 LX150 FPGA boards, and I'm sure that if someone had managed to overclock them to that extent we'd have heard about. You're not dipping them in liquid nitrogen by any chance?
Seriously guys, due diligence here.