I haven't managed to synthesize anything that performs decently on a Spartan 6 (it complains about a congested design that can't be routed), but ArtForz claims to have one of these running at 190MH/s.
Hmmmm. Supposedly the Spartan 6 lacks the long-distance routing fabric of the Virtex 6 chips, and both have a much higher ratio of logic to routing infrastructure than older generations. Of course, this isn't officially documented anywhere that I can find... FPGA manufacturers are annoyingly secretive.