I just did a scan over the 7 series datasheets to see their relative performance. Here is a summary of the important figures; I figure they might be helpful for this and any other 7-series based projects:
Everything is for the -2 speed grade Artix 7 | Kintex 7 | Virtex 7
FIFO Fmax | 460.83 | 543.77 | 543.77
DSP48E1 Fmax | 550.66 | 650.20 | 650.20
The FIFO Fmax (block ram) has been a fairly good measure of the absolute maximum frequency we can expect to see out of hashing cores. My rough estimates show that Artix 7 is likely to have a better MH/s/$ based on these figures alone, and single unit prices. However, it is difficult to tell for sure, because I suspect that the Artix is crippled in some other way. I have not check each chip's routing and CLB configurations.
I'm suprised that the kintex and vortex are equal, I would expect the virtex to be quiet a bit faster xD
but thanks, for this, this means the FPGA should put out about 1GH/s