There are already a handful of people mining altcoins with fpga´s, and they are getting insane hashrates. Only downside is that they don´t release their bitstreams to the public.
Why should they release their bitstreams to the public ? They become nothing for it !
Psst... "become" doesn't mean bekommen.
They do receive something. A nice and beefy dev fee. Look at the total ETH hashpower for example, assume 80% of it is mined using Claymore's miner and take 2% thereof (his fee).
Do the math.
Bitstreams don't work that way. Right now there is no way to capture a developer fee, since software on the PC still has to talk to the FPGA to connect the hashpower to a pool. Given all the folks who will happily recompile and distribute mining programs after disabling the fee to the unwashed masses, there is no upside. Someone will 'improve' the miner and remove the dev fee, and once the bitstream is out, there is no way to fix that.
The Allmine shell will fix that, so developers can release encrypted streams and capture a fee off of the hashrate, but today, as it exists, nobody in their right mind would do it. Until then, FPGA's will mine in big farms where they can pay for the development costs.