I think you're overestimating how finely engineered the bitcoin FPGA's would be. It's quite unlikely that the IO bus has been tuned to the barest minimum before affecting hash rates, I'd think it's much more likely that they were a) the easiest to acquire/use, and b) the cheapest. Odds are the low end FPGA's from the same manufacturer use the same bus as the high end ones, it's just a different processing unit and oscillator.

You aren't decoding video here, you're still manipulating strings. WPA has a key length of 256 bits.

All the same, buy a few different ones, try them out and let us know!