This is phenomenal that you could get the cost down so low. My only complaint was going to be the JTAG interface. But, since you've accounted for daisy-chaining 6 of them... that complaint is gone. Hopefully your JTAG signal integrity holds up from daisy-chaining 6 boards. It's touchy.
You raise an important point, although you can always crank the JTAG clock rate down to something ridiculous like 0.25Mhz. Sure, it will take a full minute or two to load the bitstream, but once that's done you've still got an order of magnitude more bandwidth than you need for loading getwork's and reading back nonces.
So, yes, signal integrity is an issue here and
I fully expect that the JTAG clock rate will need to be reduced when all six slots are full. Once I have another four boards built I will know this for sure.
But the reduced clock rate will not affect mining throughput at all -- just "boot up" time.