That reminds me - have you managed to synthesize your code for a Spartan 6? I tried it, but it bailed out early on with a cryptic message about synthesis failing and no other information I could find. Rumour has it the Spartan 6 support may be more temperamental than for earlier generations. (Not that I have an FPGA to run this on anyway!)
No, I haven't tried it as I only have a Spartan 3E 500K. I have only been looking at the specs of Spartan 6 and others, so as to find the biggest number of logic units. Thanks for the idea though, testing the synthesis in advance would help us choose the best chip.
On another note, I have been estimating how my miner performs, based on how often a solution is found. I'm getting something like 3 to 4 Mhash/s at 100 MHz, which is much better than expected, but I may just be lucky. The mining script is updated to show these estimates, though with some more work you could get actual rates from the chip.