The high end Spartan-6 has ~150K gates.
Is this type of thing cost effective for miners to buy just for mining? Nope you'd likely never pay off cost of the cluster from your mining.
Is it cost effective for someone who already owns units used for other work? Very, considering each chip only pulls ~5 watts and they're sitting idle.
Yes. 150K LU is not enough to fit an unrolled engine with internally pipelined adders. It's enough to fit _one_ unrolled engine, which you'd probably be lucky to get running at 100MHz (=100MH/s). Maybe you could do some awesome stunts, depending on the platform and somehow get two in, though I don't see it.
If it's otherwise idle capacity, then fine— it would be profitable. But you're not talking about a huge competitive advantage for anyone yet, certainly not a huge short term competitive advantage.
The bounty is laughable... A person keeping the code to themselves could profit a lot more than that and keep the competitive advantage. You may not like it but it's capitalism.
Much more and it simply becomes easier to write it myself. It's quite simple to write a SHA2-256 engine in verilog, though harder to get it going fast.
To someone who knows the tools and has the development kit handy, it's probably two days of work to get something basic going, though the skys is the limit on optimizations. I'd offer the use of hardware, but the largest programmable FPGA I own is only 27K LU, which is too small to be interesting for this.
The reality is that someone will eventually do it for love or money and reality trumps capitalism.
Moreover, shaking confidence in the security of bitcoin by securing a large private advantage wouldn't be economically sensible for anyone developing this stuff in any case.